'use strict';

const packageInfo = require('../../package.json');
const EventEmitter = require('events').EventEmitter;
const net = require('net');
const tls = require('tls');
const os = require('os');
const crypto = require('crypto');
const DataStream = require('./data-stream');
const PassThrough = require('stream').PassThrough;
const shared = require('../shared');

// default timeout values in ms
const CONNECTION_TIMEOUT = 2 * 60 * 1000; // how much to wait for the connection to be established
const SOCKET_TIMEOUT = 10 * 60 * 1000; // how much to wait for socket inactivity before disconnecting the client
const GREETING_TIMEOUT = 30 * 1000; // how much to wait after connection is established but SMTP greeting is not receieved

/**
 * Generates a SMTP connection object
 *
 * Optional options object takes the following possible properties:
 *
 *  * **port** - is the port to connect to (defaults to 587 or 465)
 *  * **host** - is the hostname or IP address to connect to (defaults to 'localhost')
 *  * **secure** - use SSL
 *  * **ignoreTLS** - ignore server support for STARTTLS
 *  * **requireTLS** - forces the client to use STARTTLS
 *  * **name** - the name of the client server
 *  * **localAddress** - outbound address to bind to (see: http://nodejs.org/api/net.html#net_net_connect_options_connectionlistener)
 *  * **greetingTimeout** - Time to wait in ms until greeting message is received from the server (defaults to 10000)
 *  * **connectionTimeout** - how many milliseconds to wait for the connection to establish
 *  * **socketTimeout** - Time of inactivity until the connection is closed (defaults to 1 hour)
 *  * **lmtp** - if true, uses LMTP instead of SMTP protocol
 *  * **logger** - bunyan compatible logger interface
 *  * **debug** - if true pass SMTP traffic to the logger
 *  * **tls** - options for createCredentials
 *  * **socket** - existing socket to use instead of creating a new one (see: http://nodejs.org/api/net.html#net_class_net_socket)
 *  * **secured** - boolean indicates that the provided socket has already been upgraded to tls
 *
 * @constructor
 * @namespace SMTP Client module
 * @param {Object} [options] Option properties
 */
class SMTPConnection extends EventEmitter {
    constructor(options) {
        super(options);

        this.id = crypto.randomBytes(8).toString('base64').replace(/\W/g, '');
        this.stage = 'init';

        this.options = options || {};

        this.secureConnection = !!this.options.secure;
        this.alreadySecured = !!this.options.secured;

        this.port = Number(this.options.port) || (this.secureConnection ? 465 : 587);
        this.host = this.options.host || 'localhost';

        if (typeof this.options.secure === 'undefined' && this.port === 465) {
            // if secure option is not set but port is 465, then default to secure
            this.secureConnection = true;
        }

        this.name = this.options.name || this._getHostname();

        this.logger = shared.getLogger(this.options, {
            component: this.options.component || 'smtp-connection',
            sid: this.id
        });

        this.customAuth = new Map();
        Object.keys(this.options.customAuth || {}).forEach(key => {
            let mapKey = (key || '').toString().trim().toUpperCase();
            if (!mapKey) {
                return;
            }
            this.customAuth.set(mapKey, this.options.customAuth[key]);
        });

        /**
         * Expose version nr, just for the reference
         * @type {String}
         */
        this.version = packageInfo.version;

        /**
         * If true, then the user is authenticated
         * @type {Boolean}
         */
        this.authenticated = false;

        /**
         * If set to true, this instance is no longer active
         * @private
         */
        this.destroyed = false;

        /**
         * Defines if the current connection is secure or not. If not,
         * STARTTLS can be used if available
         * @private
         */
        this.secure = !!this.secureConnection;

        /**
         * Store incomplete messages coming from the server
         * @private
         */
        this._remainder = '';

        /**
         * Unprocessed responses from the server
         * @type {Array}
         */
        this._responseQueue = [];

        this.lastServerResponse = false;

        /**
         * The socket connecting to the server
         * @publick
         */
        this._socket = false;

        /**
         * Lists supported auth mechanisms
         * @private
         */
        this._supportedAuth = [];

        /**
         * Set to true, if EHLO response includes "AUTH".
         * If false then authentication is not tried
         */
        this.allowsAuth = false;

        /**
         * Includes current envelope (from, to)
         * @private
         */
        this._envelope = false;

        /**
         * Lists supported extensions
         * @private
         */
        this._supportedExtensions = [];

        /**
         * Defines the maximum allowed size for a single message
         * @private
         */
        this._maxAllowedSize = 0;

        /**
         * Function queue to run if a data chunk comes from the server
         * @private
         */
        this._responseActions = [];
        this._recipientQueue = [];

        /**
         * Timeout variable for waiting the greeting
         * @private
         */
        this._greetingTimeout = false;

        /**
         * Timeout variable for waiting the connection to start
         * @private
         */
        this._connectionTimeout = false;

        /**
         * If the socket is deemed already closed
         * @private
         */
        this._destroyed = false;

        /**
         * If the socket is already being closed
         * @private
         */
        this._closing = false;

        /**
         * Callbacks for socket's listeners
         */
        this._onSocketData = chunk => this._onData(chunk);
        this._onSocketError = error => this._onError(error, 'ESOCKET', false, 'CONN');
        this._onSocketClose = () => this._onClose();
        this._onSocketEnd = () => this._onEnd();
        this._onSocketTimeout = () => this._onTimeout();
    }

    /**
     * Creates a connection to a SMTP server and sets up connection
     * listener
     */
    connect(connectCallback) {
        if (typeof connectCallback === 'function') {
            this.once('connect', () => {
                this.logger.debug(
                    {
                        tnx: 'smtp'
                    },
                    'SMTP handshake finished'
                );
                connectCallback();
            });

            const isDestroyedMessage = this._isDestroyedMessage('connect');
            if (isDestroyedMessage) {
                return connectCallback(this._formatError(isDestroyedMessage, 'ECONNECTION', false, 'CONN'));
            }
        }

        let opts = {
            port: this.port,
            host: this.host
        };

        if (this.options.localAddress) {
            opts.localAddress = this.options.localAddress;
        }

        let setupConnectionHandlers = () => {
            this._connectionTimeout = setTimeout(() => {
                this._onError('Connection timeout', 'ETIMEDOUT', false, 'CONN');
            }, this.options.connectionTimeout || CONNECTION_TIMEOUT);

            this._socket.on('error', this._onSocketError);
        };

        if (this.options.connection) {
            // connection is already opened
            this._socket = this.options.connection;
            if (this.secureConnection && !this.alreadySecured) {
                setImmediate(() =>
                    this._upgradeConnection(err => {
                        if (err) {
                            this._onError(new Error('Error initiating TLS - ' + (err.message || err)), 'ETLS', false, 'CONN');
                            return;
                        }
                        this._onConnect();
                    })
                );
            } else {
                setImmediate(() => this._onConnect());
            }
            return;
        } else if (this.options.socket) {
            // socket object is set up but not yet connected
            this._socket = this.options.socket;
            return shared.resolveHostname(opts, (err, resolved) => {
                if (err) {
                    return setImmediate(() => this._onError(err, 'EDNS', false, 'CONN'));
                }
                this.logger.debug(
                    {
                        tnx: 'dns',
                        source: opts.host,
                        resolved: resolved.host,
                        cached: !!resolved._cached
                    },
                    'Resolved %s as %s [cache %s]',
                    opts.host,
                    resolved.host,
                    resolved._cached ? 'hit' : 'miss'
                );
                Object.keys(resolved).forEach(key => {
                    if (key.charAt(0) !== '_' && resolved[key]) {
                        opts[key] = resolved[key];
                    }
                });
                try {
                    this._socket.connect(this.port, this.host, () => {
                        this._socket.setKeepAlive(true);
                        this._onConnect();
                    });
                    setupConnectionHandlers();
                } catch (E) {
                    return setImmediate(() => this._onError(E, 'ECONNECTION', false, 'CONN'));
                }
            });
        } else if (this.secureConnection) {
            // connect using tls
            if (this.options.tls) {
                Object.keys(this.options.tls).forEach(key => {
                    opts[key] = this.options.tls[key];
                });
            }
            return shared.resolveHostname(opts, (err, resolved) => {
                if (err) {
                    return setImmediate(() => this._onError(err, 'EDNS', false, 'CONN'));
                }
                this.logger.debug(
                    {
                        tnx: 'dns',
                        source: opts.host,
                        resolved: resolved.host,
                        cached: !!resolved._cached
                    },
                    'Resolved %s as %s [cache %s]',
                    opts.host,
                    resolved.host,
                    resolved._cached ? 'hit' : 'miss'
                );
                Object.keys(resolved).forEach(key => {
                    if (key.charAt(0) !== '_' && resolved[key]) {
                        opts[key] = resolved[key];
                    }
                });
                try {
                    this._socket = tls.connect(opts, () => {
                        this._socket.setKeepAlive(true);
                        this._onConnect();
                    });
                    setupConnectionHandlers();
                } catch (E) {
                    return setImmediate(() => this._onError(E, 'ECONNECTION', false, 'CONN'));
                }
            });
        } else {
            // connect using plaintext
            return shared.resolveHostname(opts, (err, resolved) => {
                if (err) {
                    return setImmediate(() => this._onError(err, 'EDNS', false, 'CONN'));
                }
                this.logger.debug(
                    {
                        tnx: 'dns',
                        source: opts.host,
                        resolved: resolved.host,
                        cached: !!resolved._cached
                    },
                    'Resolved %s as %s [cache %s]',
                    opts.host,
                    resolved.host,
                    resolved._cached ? 'hit' : 'miss'
                );
                Object.keys(resolved).forEach(key => {
                    if (key.charAt(0) !== '_' && resolved[key]) {
                        opts[key] = resolved[key];
                    }
                });
                try {
                    this._socket = net.connect(opts, () => {
                        this._socket.setKeepAlive(true);
                        this._onConnect();
                    });
                    setupConnectionHandlers();
                } catch (E) {
                    return setImmediate(() => this._onError(E, 'ECONNECTION', false, 'CONN'));
                }
            });
        }
    }

    /**
     * Sends QUIT
     */
    quit() {
        this._sendCommand('QUIT');
        this._responseActions.push(this.close);
    }

    /**
     * Closes the connection to the server
     */
    close() {
        clearTimeout(this._connectionTimeout);
        clearTimeout(this._greetingTimeout);
        this._responseActions = [];

        // allow to run this function only once
        if (this._closing) {
            return;
        }
        this._closing = true;

        let closeMethod = 'end';

        if (this.stage === 'init') {
            // Close the socket immediately when connection timed out
            closeMethod = 'destroy';
        }

        this.logger.debug(
            {
                tnx: 'smtp'
            },
            'Closing connection to the server using "%s"',
            closeMethod
        );

        let socket = (this._socket && this._socket.socket) || this._socket;

        if (socket && !socket.destroyed) {
            try {
                this._socket[closeMethod]();
            } catch (E) {
                // just ignore
            }
        }

        this._destroy();
    }

    /**
     * Authenticate user
     */
    login(authData, callback) {
        const isDestroyedMessage = this._isDestroyedMessage('login');
        if (isDestroyedMessage) {
            return callback(this._formatError(isDestroyedMessage, 'ECONNECTION', false, 'API'));
        }

        this._auth = authData || {};
        // Select SASL authentication method
        this._authMethod = (this._auth.method || '').toString().trim().toUpperCase() || false;

        if (!this._authMethod && this._auth.oauth2 && !this._auth.credentials) {
            this._authMethod = 'XOAUTH2';
        } else if (!this._authMethod || (this._authMethod === 'XOAUTH2' && !this._auth.oauth2)) {
            // use first supported
            this._authMethod = (this._supportedAuth[0] || 'PLAIN').toUpperCase().trim();
        }

        if (this._authMethod !== 'XOAUTH2' && (!this._auth.credentials || !this._auth.credentials.user || !this._auth.credentials.pass)) {
            if (this._auth.user && this._auth.pass) {
                this._auth.credentials = {
                    user: this._auth.user,
                    pass: this._auth.pass,
                    options: this._auth.options
                };
            } else {
                return callback(this._formatError('Missing credentials for "' + this._authMethod + '"', 'EAUTH', false, 'API'));
            }
        }

        if (this.customAuth.has(this._authMethod)) {
            let handler = this.customAuth.get(this._authMethod);
            let lastResponse;
            let returned = false;

            let resolve = () => {
                if (returned) {
                    return;
                }
                returned = true;
                this.logger.info(
                    {
                        tnx: 'smtp',
                        username: this._auth.user,
                        action: 'authenticated',
                        method: this._authMethod
                    },
                    'User %s authenticated',
                    JSON.stringify(this._auth.user)
                );
                this.authenticated = true;
                callback(null, true);
            };

            let reject = err => {
                if (returned) {
                    return;
                }
                returned = true;
                callback(this._formatError(err, 'EAUTH', lastResponse, 'AUTH ' + this._authMethod));
            };

            let handlerResponse = handler({
                auth: this._auth,
                method: this._authMethod,

                extensions: [].concat(this._supportedExtensions),
                authMethods: [].concat(this._supportedAuth),
                maxAllowedSize: this._maxAllowedSize || false,

                sendCommand: (cmd, done) => {
                    let promise;

                    if (!done) {
                        promise = new Promise((resolve, reject) => {
                            done = shared.callbackPromise(resolve, reject);
                        });
                    }

                    this._responseActions.push(str => {
                        lastResponse = str;

                        let codes = str.match(/^(\d+)(?:\s(\d+\.\d+\.\d+))?\s/);
                        let data = {
                            command: cmd,
                            response: str
                        };
                        if (codes) {
                            data.status = Number(codes[1]) || 0;
                            if (codes[2]) {
                                data.code = codes[2];
                            }
                            data.text = str.substr(codes[0].length);
                        } else {
                            data.text = str;
                            data.status = 0; // just in case we need to perform numeric comparisons
                        }
                        done(null, data);
                    });
                    setImmediate(() => this._sendCommand(cmd));

                    return promise;
                },

                resolve,
                reject
            });

            if (handlerResponse && typeof handlerResponse.catch === 'function') {
                // a promise was returned
                handlerResponse.then(resolve).catch(reject);
            }

            return;
        }

        switch (this._authMethod) {
            case 'XOAUTH2':
                this._handleXOauth2Token(false, callback);
                return;
            case 'LOGIN':
                this._responseActions.push(str => {
                    this._actionAUTH_LOGIN_USER(str, callback);
                });
                this._sendCommand('AUTH LOGIN');
                return;
            case 'PLAIN':
                this._responseActions.push(str => {
                    this._actionAUTHComplete(str, callback);
                });
                this._sendCommand(
                    'AUTH PLAIN ' +
                        Buffer.from(
                            //this._auth.user+'\u0000'+
                            '\u0000' + // skip authorization identity as it causes problems with some servers
                                this._auth.credentials.user +
                                '\u0000' +
                                this._auth.credentials.pass,
                            'utf-8'
                        ).toString('base64')
                );
                return;
            case 'CRAM-MD5':
                this._responseActions.push(str => {
                    this._actionAUTH_CRAM_MD5(str, callback);
                });
                this._sendCommand('AUTH CRAM-MD5');
                return;
        }

        return callback(this._formatError('Unknown authentication method "' + this._authMethod + '"', 'EAUTH', false, 'API'));
    }

    /**
     * Sends a message
     *
     * @param {Object} envelope Envelope object, {from: addr, to: [addr]}
     * @param {Object} message String, Buffer or a Stream
     * @param {Function} callback Callback to return once sending is completed
     */
    send(envelope, message, done) {
        if (!message) {
            return done(this._formatError('Empty message', 'EMESSAGE', false, 'API'));
        }

        const isDestroyedMessage = this._isDestroyedMessage('send message');
        if (isDestroyedMessage) {
            return done(this._formatError(isDestroyedMessage, 'ECONNECTION', false, 'API'));
        }

        // reject larger messages than allowed
        if (this._maxAllowedSize && envelope.size > this._maxAllowedSize) {
            return setImmediate(() => {
                done(this._formatError('Message size larger than allowed ' + this._maxAllowedSize, 'EMESSAGE', false, 'MAIL FROM'));
            });
        }

        // ensure that callback is only called once
        let returned = false;
        let callback = function () {
            if (returned) {
                return;
            }
            returned = true;

            done(...arguments);
        };

        if (typeof message.on === 'function') {
            message.on('error', err => callback(this._formatError(err, 'ESTREAM', false, 'API')));
        }

        let startTime = Date.now();
        this._setEnvelope(envelope, (err, info) => {
            if (err) {
                return callback(err);
            }
            let envelopeTime = Date.now();
            let stream = this._createSendStream((err, str) => {
                if (err) {
                    return callback(err);
                }

                info.envelopeTime = envelopeTime - startTime;
                info.messageTime = Date.now() - envelopeTime;
                info.messageSize = stream.outByteCount;
                info.response = str;

                return callback(null, info);
            });
            if (typeof message.pipe === 'function') {
                message.pipe(stream);
            } else {
                stream.write(message);
                stream.end();
            }
        });
    }

    /**
     * Resets connection state
     *
     * @param {Function} callback Callback to return once connection is reset
     */
    reset(callback) {
        this._sendCommand('RSET');
        this._responseActions.push(str => {
            if (str.charAt(0) !== '2') {
                return callback(this._formatError('Could not reset session state. response=' + str, 'EPROTOCOL', str, 'RSET'));
            }
            this._envelope = false;
            return callback(null, true);
        });
    }

    /**
     * Connection listener that is run when the connection to
     * the server is opened
     *
     * @event
     */
    _onConnect() {
        clearTimeout(this._connectionTimeout);

        this.logger.info(
            {
                tnx: 'network',
                localAddress: this._socket.localAddress,
                localPort: this._socket.localPort,
                remoteAddress: this._socket.remoteAddress,
                remotePort: this._socket.remotePort
            },
            '%s established to %s:%s',
            this.secure ? 'Secure connection' : 'Connection',
            this._socket.remoteAddress,
            this._socket.remotePort
        );

        if (this._destroyed) {
            // Connection was established after we already had canceled it
            this.close();
            return;
        }

        this.stage = 'connected';

        // clear existing listeners for the socket
        this._socket.removeListener('data', this._onSocketData);
        this._socket.removeListener('timeout', this._onSocketTimeout);
        this._socket.removeListener('close', this._onSocketClose);
        this._socket.removeListener('end', this._onSocketEnd);

        this._socket.on('data', this._onSocketData);
        this._socket.once('close', this._onSocketClose);
        this._socket.once('end', this._onSocketEnd);

        this._socket.setTimeout(this.options.socketTimeout || SOCKET_TIMEOUT);
        this._socket.on('timeout', this._onSocketTimeout);

        this._greetingTimeout = setTimeout(() => {
            // if still waiting for greeting, give up
            if (this._socket && !this._destroyed && this._responseActions[0] === this._actionGreeting) {
                this._onError('Greeting never received', 'ETIMEDOUT', false, 'CONN');
            }
        }, this.options.greetingTimeout || GREETING_TIMEOUT);

        this._responseActions.push(this._actionGreeting);

        // we have a 'data' listener set up so resume socket if it was paused
        this._socket.resume();
    }

    /**
     * 'data' listener for data coming from the server
     *
     * @event
     * @param {Buffer} chunk Data chunk coming from the server
     */
    _onData(chunk) {
        if (this._destroyed || !chunk || !chunk.length) {
            return;
        }

        let data = (chunk || '').toString('binary');
        let lines = (this._remainder + data).split(/\r?\n/);
        let lastline;

        this._remainder = lines.pop();

        for (let i = 0, len = lines.length; i < len; i++) {
            if (this._responseQueue.length) {
                lastline = this._responseQueue[this._responseQueue.length - 1];
                if (/^\d+-/.test(lastline.split('\n').pop())) {
                    this._responseQueue[this._responseQueue.length - 1] += '\n' + lines[i];
                    continue;
                }
            }
            this._responseQueue.push(lines[i]);
        }

        if (this._responseQueue.length) {
            lastline = this._responseQueue[this._responseQueue.length - 1];
            if (/^\d+-/.test(lastline.split('\n').pop())) {
                return;
            }
        }

        this._processResponse();
    }

    /**
     * 'error' listener for the socket
     *
     * @event
     * @param {Error} err Error object
     * @param {String} type Error name
     */
    _onError(err, type, data, command) {
        clearTimeout(this._connectionTimeout);
        clearTimeout(this._greetingTimeout);

        if (this._destroyed) {
            // just ignore, already closed
            // this might happen when a socket is canceled because of reached timeout
            // but the socket timeout error itself receives only after
            return;
        }

        err = this._formatError(err, type, data, command);

        this.logger.error(data, err.message);

        this.emit('error', err);
        this.close();
    }

    _formatError(message, type, response, command) {
        let err;

        if (/Error\]$/i.test(Object.prototype.toString.call(message))) {
            err = message;
        } else {
            err = new Error(message);
        }

        if (type && type !== 'Error') {
            err.code = type;
        }

        if (response) {
            err.response = response;
            err.message += ': ' + response;
        }

        let responseCode = (typeof response === 'string' && Number((response.match(/^\d+/) || [])[0])) || false;
        if (responseCode) {
            err.responseCode = responseCode;
        }

        if (command) {
            err.command = command;
        }

        return err;
    }

    /**
     * 'close' listener for the socket
     *
     * @event
     */
    _onClose() {
        this.logger.info(
            {
                tnx: 'network'
            },
            'Connection closed'
        );

        if (this.upgrading && !this._destroyed) {
            return this._onError(new Error('Connection closed unexpectedly'), 'ETLS', false, 'CONN');
        } else if (![this._actionGreeting, this.close].includes(this._responseActions[0]) && !this._destroyed) {
            return this._onError(new Error('Connection closed unexpectedly'), 'ECONNECTION', false, 'CONN');
        }

        this._destroy();
    }

    /**
     * 'end' listener for the socket
     *
     * @event
     */
    _onEnd() {
        if (this._socket && !this._socket.destroyed) {
            this._socket.destroy();
        }
    }

    /**
     * 'timeout' listener for the socket
     *
     * @event
     */
    _onTimeout() {
        return this._onError(new Error('Timeout'), 'ETIMEDOUT', false, 'CONN');
    }

    /**
     * Destroys the client, emits 'end'
     */
    _destroy() {
        if (this._destroyed) {
            return;
        }
        this._destroyed = true;
        this.emit('end');
    }

    /**
     * Upgrades the connection to TLS
     *
     * @param {Function} callback Callback function to run when the connection
     *        has been secured
     */
    _upgradeConnection(callback) {
        // do not remove all listeners or it breaks node v0.10 as there's
        // apparently a 'finish' event set that would be cleared as well

        // we can safely keep 'error', 'end', 'close' etc. events
        this._socket.removeListener('data', this._onSocketData); // incoming data is going to be gibberish from this point onwards
        this._socket.removeListener('timeout', this._onSocketTimeout); // timeout will be re-set for the new socket object

        let socketPlain = this._socket;
        let opts = {
            socket: this._socket,
            host: this.host
        };

        Object.keys(this.options.tls || {}).forEach(key => {
            opts[key] = this.options.tls[key];
        });

        this.upgrading = true;
        // tls.connect is not an asynchronous function however it may still throw errors and requires to be wrapped with try/catch
        try {
            this._socket = tls.connect(opts, () => {
                this.secure = true;
                this.upgrading = false;
                this._socket.on('data', this._onSocketData);

                socketPlain.removeListener('close', this._onSocketClose);
                socketPlain.removeListener('end', this._onSocketEnd);

                return callback(null, true);
            });
        } catch (err) {
            return callback(err);
        }

        this._socket.on('error', this._onSocketError);
        this._socket.once('close', this._onSocketClose);
        this._socket.once('end', this._onSocketEnd);

        this._socket.setTimeout(this.options.socketTimeout || SOCKET_TIMEOUT); // 10 min.
        this._socket.on('timeout', this._onSocketTimeout);

        // resume in case the socket was paused
        socketPlain.resume();
    }

    /**
     * Processes queued responses from the server
     *
     * @param {Boolean} force If true, ignores _processing flag
     */
    _processResponse() {
        if (!this._responseQueue.length) {
            return false;
        }

        let str = (this.lastServerResponse = (this._responseQueue.shift() || '').toString());

        if (/^\d+-/.test(str.split('\n').pop())) {
            // keep waiting for the final part of multiline response
            return;
        }

        if (this.options.debug || this.options.transactionLog) {
            this.logger.debug(
                {
                    tnx: 'server'
                },
                str.replace(/\r?\n$/, '')
            );
        }

        if (!str.trim()) {
            // skip unexpected empty lines
            setImmediate(() => this._processResponse(true));
        }

        let action = this._responseActions.shift();

        if (typeof action === 'function') {
            action.call(this, str);
            setImmediate(() => this._processResponse(true));
        } else {
            return this._onError(new Error('Unexpected Response'), 'EPROTOCOL', str, 'CONN');
        }
    }

    /**
     * Send a command to the server, append \r\n
     *
     * @param {String} str String to be sent to the server
     */
    _sendCommand(str) {
        if (this._destroyed) {
            // Connection already closed, can't send any more data
            return;
        }

        if (this._socket.destroyed) {
            return this.close();
        }

        if (this.options.debug || this.options.transactionLog) {
            this.logger.debug(
                {
                    tnx: 'client'
                },
                (str || '').toString().replace(/\r?\n$/, '')
            );
        }

        this._socket.write(Buffer.from(str + '\r\n', 'utf-8'));
    }

    /**
     * Initiates a new message by submitting envelope data, starting with
     * MAIL FROM: command
     *
     * @param {Object} envelope Envelope object in the form of
     *        {from:'...', to:['...']}
     *        or
     *        {from:{address:'...',name:'...'}, to:[address:'...',name:'...']}
     */
    _setEnvelope(envelope, callback) {
        let args = [];
        let useSmtpUtf8 = false;

        this._envelope = envelope || {};
        this._envelope.from = ((this._envelope.from && this._envelope.from.address) || this._envelope.from || '').toString().trim();

        this._envelope.to = [].concat(this._envelope.to || []).map(to => ((to && to.address) || to || '').toString().trim());

        if (!this._envelope.to.length) {
            return callback(this._formatError('No recipients defined', 'EENVELOPE', false, 'API'));
        }

        if (this._envelope.from && /[\r\n<>]/.test(this._envelope.from)) {
            return callback(this._formatError('Invalid sender ' + JSON.stringify(this._envelope.from), 'EENVELOPE', false, 'API'));
        }

        // check if the sender address uses only ASCII characters,
        // otherwise require usage of SMTPUTF8 extension
        if (/[\x80-\uFFFF]/.test(this._envelope.from)) {
            useSmtpUtf8 = true;
        }

        for (let i = 0, len = this._envelope.to.length; i < len; i++) {
            if (!this._envelope.to[i] || /[\r\n<>]/.test(this._envelope.to[i])) {
                return callback(this._formatError('Invalid recipient ' + JSON.stringify(this._envelope.to[i]), 'EENVELOPE', false, 'API'));
            }

            // check if the recipients addresses use only ASCII characters,
            // otherwise require usage of SMTPUTF8 extension
            if (/[\x80-\uFFFF]/.test(this._envelope.to[i])) {
                useSmtpUtf8 = true;
            }
        }

        // clone the recipients array for latter manipulation
        this._envelope.rcptQueue = JSON.parse(JSON.stringify(this._envelope.to || []));
        this._envelope.rejected = [];
        this._envelope.rejectedErrors = [];
        this._envelope.accepted = [];

        if (this._envelope.dsn) {
            try {
                this._envelope.dsn = this._setDsnEnvelope(this._envelope.dsn);
            } catch (err) {
                return callback(this._formatError('Invalid DSN ' + err.message, 'EENVELOPE', false, 'API'));
            }
        }

        this._responseActions.push(str => {
            this._actionMAIL(str, callback);
        });

        // If the server supports SMTPUTF8 and the envelope includes an internationalized
        // email address then append SMTPUTF8 keyword to the MAIL FROM command
        if (useSmtpUtf8 && this._supportedExtensions.includes('SMTPUTF8')) {
            args.push('SMTPUTF8');
            this._usingSmtpUtf8 = true;
        }

        // If the server supports 8BITMIME and the message might contain non-ascii bytes
        // then append the 8BITMIME keyword to the MAIL FROM command
        if (this._envelope.use8BitMime && this._supportedExtensions.includes('8BITMIME')) {
            args.push('BODY=8BITMIME');
            this._using8BitMime = true;
        }

        if (this._envelope.size && this._supportedExtensions.includes('SIZE')) {
            args.push('SIZE=' + this._envelope.size);
        }

        // If the server supports DSN and the envelope includes an DSN prop
        // then append DSN params to the MAIL FROM command
        if (this._envelope.dsn && this._supportedExtensions.includes('DSN')) {
            if (this._envelope.dsn.ret) {
                args.push('RET=' + shared.encodeXText(this._envelope.dsn.ret));
            }
            if (this._envelope.dsn.envid) {
                args.push('ENVID=' + shared.encodeXText(this._envelope.dsn.envid));
            }
        }

        this._sendCommand('MAIL FROM:<' + this._envelope.from + '>' + (args.length ? ' ' + args.join(' ') : ''));
    }

    _setDsnEnvelope(params) {
        let ret = (params.ret || params.return || '').toString().toUpperCase() || null;
        if (ret) {
            switch (ret) {
                case 'HDRS':
                case 'HEADERS':
                    ret = 'HDRS';
                    break;
                case 'FULL':
                case 'BODY':
                    ret = 'FULL';
                    break;
            }
        }

        if (ret && !['FULL', 'HDRS'].includes(ret)) {
            throw new Error('ret: ' + JSON.stringify(ret));
        }

        let envid = (params.envid || params.id || '').toString() || null;

        let notify = params.notify || null;
        if (notify) {
            if (typeof notify === 'string') {
                notify = notify.split(',');
            }
            notify = notify.map(n => n.trim().toUpperCase());
            let validNotify = ['NEVER', 'SUCCESS', 'FAILURE', 'DELAY'];
            let invaliNotify = notify.filter(n => !validNotify.includes(n));
            if (invaliNotify.length || (notify.length > 1 && notify.includes('NEVER'))) {
                throw new Error('notify: ' + JSON.stringify(notify.join(',')));
            }
            notify = notify.join(',');
        }

        let orcpt = (params.orcpt || params.recipient || '').toString() || null;
        if (orcpt && orcpt.indexOf(';') < 0) {
            orcpt = 'rfc822;' + orcpt;
        }

        return {
            ret,
            envid,
            notify,
            orcpt
        };
    }

    _getDsnRcptToArgs() {
        let args = [];
        // If the server supports DSN and the envelope includes an DSN prop
        // then append DSN params to the RCPT TO command
        if (this._envelope.dsn && this._supportedExtensions.includes('DSN')) {
            if (this._envelope.dsn.notify) {
                args.push('NOTIFY=' + shared.encodeXText(this._envelope.dsn.notify));
            }
            if (this._envelope.dsn.orcpt) {
                args.push('ORCPT=' + shared.encodeXText(this._envelope.dsn.orcpt));
            }
        }
        return args.length ? ' ' + args.join(' ') : '';
    }

    _createSendStream(callback) {
        let dataStream = new DataStream();
        let logStream;

        if (this.options.lmtp) {
            this._envelope.accepted.forEach((recipient, i) => {
                let final = i === this._envelope.accepted.length - 1;
                this._responseActions.push(str => {
                    this._actionLMTPStream(recipient, final, str, callback);
                });
            });
        } else {
            this._responseActions.push(str => {
                this._actionSMTPStream(str, callback);
            });
        }

        dataStream.pipe(this._socket, {
            end: false
        });

        if (this.options.debug) {
            logStream = new PassThrough();
            logStream.on('readable', () => {
                let chunk;
                while ((chunk = logStream.read())) {
                    this.logger.debug(
                        {
                            tnx: 'message'
                        },
                        chunk.toString('binary').replace(/\r?\n$/, '')
                    );
                }
            });
            dataStream.pipe(logStream);
        }

        dataStream.once('end', () => {
            this.logger.info(
                {
                    tnx: 'message',
                    inByteCount: dataStream.inByteCount,
                    outByteCount: dataStream.outByteCount
                },
                '<%s bytes encoded mime message (source size %s bytes)>',
                dataStream.outByteCount,
                dataStream.inByteCount
            );
        });

        return dataStream;
    }

    /** ACTIONS **/

    /**
     * Will be run after the connection is created and the server sends
     * a greeting. If the incoming message starts with 220 initiate
     * SMTP session by sending EHLO command
     *
     * @param {String} str Message from the server
     */
    _actionGreeting(str) {
        clearTimeout(this._greetingTimeout);

        if (str.substr(0, 3) !== '220') {
            this._onError(new Error('Invalid greeting. response=' + str), 'EPROTOCOL', str, 'CONN');
            return;
        }

        if (this.options.lmtp) {
            this._responseActions.push(this._actionLHLO);
            this._sendCommand('LHLO ' + this.name);
        } else {
            this._responseActions.push(this._actionEHLO);
            this._sendCommand('EHLO ' + this.name);
        }
    }

    /**
     * Handles server response for LHLO command. If it yielded in
     * error, emit 'error', otherwise treat this as an EHLO response
     *
     * @param {String} str Message from the server
     */
    _actionLHLO(str) {
        if (str.charAt(0) !== '2') {
            this._onError(new Error('Invalid LHLO. response=' + str), 'EPROTOCOL', str, 'LHLO');
            return;
        }

        this._actionEHLO(str);
    }

    /**
     * Handles server response for EHLO command. If it yielded in
     * error, try HELO instead, otherwise initiate TLS negotiation
     * if STARTTLS is supported by the server or move into the
     * authentication phase.
     *
     * @param {String} str Message from the server
     */
    _actionEHLO(str) {
        let match;

        if (str.substr(0, 3) === '421') {
            this._onError(new Error('Server terminates connection. response=' + str), 'ECONNECTION', str, 'EHLO');
            return;
        }

        if (str.charAt(0) !== '2') {
            if (this.options.requireTLS) {
                this._onError(new Error('EHLO failed but HELO does not support required STARTTLS. response=' + str), 'ECONNECTION', str, 'EHLO');
                return;
            }

            // Try HELO instead
            this._responseActions.push(this._actionHELO);
            this._sendCommand('HELO ' + this.name);
            return;
        }

        // Detect if the server supports STARTTLS
        if (!this.secure && !this.options.ignoreTLS && (/[ -]STARTTLS\b/im.test(str) || this.options.requireTLS)) {
            this._sendCommand('STARTTLS');
            this._responseActions.push(this._actionSTARTTLS);
            return;
        }

        // Detect if the server supports SMTPUTF8
        if (/[ -]SMTPUTF8\b/im.test(str)) {
            this._supportedExtensions.push('SMTPUTF8');
        }

        // Detect if the server supports DSN
        if (/[ -]DSN\b/im.test(str)) {
            this._supportedExtensions.push('DSN');
        }

        // Detect if the server supports 8BITMIME
        if (/[ -]8BITMIME\b/im.test(str)) {
            this._supportedExtensions.push('8BITMIME');
        }

        // Detect if the server supports PIPELINING
        if (/[ -]PIPELINING\b/im.test(str)) {
            this._supportedExtensions.push('PIPELINING');
        }

        // Detect if the server supports AUTH
        if (/[ -]AUTH\b/i.test(str)) {
            this.allowsAuth = true;
        }

        // Detect if the server supports PLAIN auth
        if (/[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)PLAIN/i.test(str)) {
            this._supportedAuth.push('PLAIN');
        }

        // Detect if the server supports LOGIN auth
        if (/[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)LOGIN/i.test(str)) {
            this._supportedAuth.push('LOGIN');
        }

        // Detect if the server supports CRAM-MD5 auth
        if (/[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)CRAM-MD5/i.test(str)) {
            this._supportedAuth.push('CRAM-MD5');
        }

        // Detect if the server supports XOAUTH2 auth
        if (/[ -]AUTH(?:(\s+|=)[^\n]*\s+|\s+|=)XOAUTH2/i.test(str)) {
            this._supportedAuth.push('XOAUTH2');
        }

        // Detect if the server supports SIZE extensions (and the max allowed size)
        if ((match = str.match(/[ -]SIZE(?:[ \t]+(\d+))?/im))) {
            this._supportedExtensions.push('SIZE');
            this._maxAllowedSize = Number(match[1]) || 0;
        }

        this.emit('connect');
    }

    /**
     * Handles server response for HELO command. If it yielded in
     * error, emit 'error', otherwise move into the authentication phase.
     *
     * @param {String} str Message from the server
     */
    _actionHELO(str) {
        if (str.charAt(0) !== '2') {
            this._onError(new Error('Invalid HELO. response=' + str), 'EPROTOCOL', str, 'HELO');
            return;
        }

        // assume that authentication is enabled (most probably is not though)
        this.allowsAuth = true;

        this.emit('connect');
    }

    /**
     * Handles server response for STARTTLS command. If there's an error
     * try HELO instead, otherwise initiate TLS upgrade. If the upgrade
     * succeedes restart the EHLO
     *
     * @param {String} str Message from the server
     */
    _actionSTARTTLS(str) {
        if (str.charAt(0) !== '2') {
            if (this.options.opportunisticTLS) {
                this.logger.info(
                    {
                        tnx: 'smtp'
                    },
                    'Failed STARTTLS upgrade, continuing unencrypted'
                );
                return this.emit('connect');
            }
            this._onError(new Error('Error upgrading connection with STARTTLS'), 'ETLS', str, 'STARTTLS');
            return;
        }

        this._upgradeConnection((err, secured) => {
            if (err) {
                this._onError(new Error('Error initiating TLS - ' + (err.message || err)), 'ETLS', false, 'STARTTLS');
                return;
            }

            this.logger.info(
                {
                    tnx: 'smtp'
                },
                'Connection upgraded with STARTTLS'
            );

            if (secured) {
                // restart session
                if (this.options.lmtp) {
                    this._responseActions.push(this._actionLHLO);
                    this._sendCommand('LHLO ' + this.name);
                } else {
                    this._responseActions.push(this._actionEHLO);
                    this._sendCommand('EHLO ' + this.name);
                }
            } else {
                this.emit('connect');
            }
        });
    }

    /**
     * Handle the response for AUTH LOGIN command. We are expecting
     * '334 VXNlcm5hbWU6' (base64 for 'Username:'). Data to be sent as
     * response needs to be base64 encoded username. We do not need
     * exact match but settle with 334 response in general as some
     * hosts invalidly use a longer message than VXNlcm5hbWU6
     *
     * @param {String} str Message from the server
     */
    _actionAUTH_LOGIN_USER(str, callback) {
        if (!/^334[ -]/.test(str)) {
            // expecting '334 VXNlcm5hbWU6'
            callback(this._formatError('Invalid login sequence while waiting for "334 VXNlcm5hbWU6"', 'EAUTH', str, 'AUTH LOGIN'));
            return;
        }

        this._responseActions.push(str => {
            this._actionAUTH_LOGIN_PASS(str, callback);
        });

        this._sendCommand(Buffer.from(this._auth.credentials.user + '', 'utf-8').toString('base64'));
    }

    /**
     * Handle the response for AUTH CRAM-MD5 command. We are expecting
     * '334 <challenge string>'. Data to be sent as response needs to be
     * base64 decoded challenge string, MD5 hashed using the password as
     * a HMAC key, prefixed by the username and a space, and finally all
     * base64 encoded again.
     *
     * @param {String} str Message from the server
     */
    _actionAUTH_CRAM_MD5(str, callback) {
        let challengeMatch = str.match(/^334\s+(.+)$/);
        let challengeString = '';

        if (!challengeMatch) {
            return callback(this._formatError('Invalid login sequence while waiting for server challenge string', 'EAUTH', str, 'AUTH CRAM-MD5'));
        } else {
            challengeString = challengeMatch[1];
        }

        // Decode from base64
        let base64decoded = Buffer.from(challengeString, 'base64').toString('ascii'),
            hmac_md5 = crypto.createHmac('md5', this._auth.credentials.pass);

        hmac_md5.update(base64decoded);

        let hex_hmac = hmac_md5.digest('hex');
        let prepended = this._auth.credentials.user + ' ' + hex_hmac;

        this._responseActions.push(str => {
            this._actionAUTH_CRAM_MD5_PASS(str, callback);
        });

        this._sendCommand(Buffer.from(prepended).toString('base64'));
    }

    /**
     * Handles the response to CRAM-MD5 authentication, if there's no error,
     * the user can be considered logged in. Start waiting for a message to send
     *
     * @param {String} str Message from the server
     */
    _actionAUTH_CRAM_MD5_PASS(str, callback) {
        if (!str.match(/^235\s+/)) {
            return callback(this._formatError('Invalid login sequence while waiting for "235"', 'EAUTH', str, 'AUTH CRAM-MD5'));
        }

        this.logger.info(
            {
                tnx: 'smtp',
                username: this._auth.user,
                action: 'authenticated',
                method: this._authMethod
            },
            'User %s authenticated',
            JSON.stringify(this._auth.user)
        );
        this.authenticated = true;
        callback(null, true);
    }

    /**
     * Handle the response for AUTH LOGIN command. We are expecting
     * '334 UGFzc3dvcmQ6' (base64 for 'Password:'). Data to be sent as
     * response needs to be base64 encoded password.
     *
     * @param {String} str Message from the server
     */
    _actionAUTH_LOGIN_PASS(str, callback) {
        if (!/^334[ -]/.test(str)) {
            // expecting '334 UGFzc3dvcmQ6'
            return callback(this._formatError('Invalid login sequence while waiting for "334 UGFzc3dvcmQ6"', 'EAUTH', str, 'AUTH LOGIN'));
        }

        this._responseActions.push(str => {
            this._actionAUTHComplete(str, callback);
        });

        this._sendCommand(Buffer.from(this._auth.credentials.pass + '', 'utf-8').toString('base64'));
    }

    /**
     * Handles the response for authentication, if there's no error,
     * the user can be considered logged in. Start waiting for a message to send
     *
     * @param {String} str Message from the server
     */
    _actionAUTHComplete(str, isRetry, callback) {
        if (!callback && typeof isRetry === 'function') {
            callback = isRetry;
            isRetry = false;
        }

        if (str.substr(0, 3) === '334') {
            this._responseActions.push(str => {
                if (isRetry || this._authMethod !== 'XOAUTH2') {
                    this._actionAUTHComplete(str, true, callback);
                } else {
                    // fetch a new OAuth2 access token
                    setImmediate(() => this._handleXOauth2Token(true, callback));
                }
            });
            this._sendCommand('');
            return;
        }

        if (str.charAt(0) !== '2') {
            this.logger.info(
                {
                    tnx: 'smtp',
                    username: this._auth.user,
                    action: 'authfail',
                    method: this._authMethod
                },
                'User %s failed to authenticate',
                JSON.stringify(this._auth.user)
            );
            return callback(this._formatError('Invalid login', 'EAUTH', str, 'AUTH ' + this._authMethod));
        }

        this.logger.info(
            {
                tnx: 'smtp',
                username: this._auth.user,
                action: 'authenticated',
                method: this._authMethod
            },
            'User %s authenticated',
            JSON.stringify(this._auth.user)
        );
        this.authenticated = true;
        callback(null, true);
    }

    /**
     * Handle response for a MAIL FROM: command
     *
     * @param {String} str Message from the server
     */
    _actionMAIL(str, callback) {
        let message, curRecipient;
        if (Number(str.charAt(0)) !== 2) {
            if (this._usingSmtpUtf8 && /^550 /.test(str) && /[\x80-\uFFFF]/.test(this._envelope.from)) {
                message = 'Internationalized mailbox name not allowed';
            } else {
                message = 'Mail command failed';
            }
            return callback(this._formatError(message, 'EENVELOPE', str, 'MAIL FROM'));
        }

        if (!this._envelope.rcptQueue.length) {
            return callback(this._formatError('Can\x27t send mail - no recipients defined', 'EENVELOPE', false, 'API'));
        } else {
            this._recipientQueue = [];

            if (this._supportedExtensions.includes('PIPELINING')) {
                while (this._envelope.rcptQueue.length) {
                    curRecipient = this._envelope.rcptQueue.shift();
                    this._recipientQueue.push(curRecipient);
                    this._responseActions.push(str => {
                        this._actionRCPT(str, callback);
                    });
                    this._sendCommand('RCPT TO:<' + curRecipient + '>' + this._getDsnRcptToArgs());
                }
            } else {
                curRecipient = this._envelope.rcptQueue.shift();
                this._recipientQueue.push(curRecipient);
                this._responseActions.push(str => {
                    this._actionRCPT(str, callback);
                });
                this._sendCommand('RCPT TO:<' + curRecipient + '>' + this._getDsnRcptToArgs());
            }
        }
    }

    /**
     * Handle response for a RCPT TO: command
     *
     * @param {String} str Message from the server
     */
    _actionRCPT(str, callback) {
        let message,
            err,
            curRecipient = this._recipientQueue.shift();
        if (Number(str.charAt(0)) !== 2) {
            // this is a soft error
            if (this._usingSmtpUtf8 && /^553 /.test(str) && /[\x80-\uFFFF]/.test(curRecipient)) {
                message = 'Internationalized mailbox name not allowed';
            } else {
                message = 'Recipient command failed';
            }
            this._envelope.rejected.push(curRecipient);
            // store error for the failed recipient
            err = this._formatError(message, 'EENVELOPE', str, 'RCPT TO');
            err.recipient = curRecipient;
            this._envelope.rejectedErrors.push(err);
        } else {
            this._envelope.accepted.push(curRecipient);
        }

        if (!this._envelope.rcptQueue.length && !this._recipientQueue.length) {
            if (this._envelope.rejected.length < this._envelope.to.length) {
                this._responseActions.push(str => {
                    this._actionDATA(str, callback);
                });
                this._sendCommand('DATA');
            } else {
                err = this._formatError('Can\x27t send mail - all recipients were rejected', 'EENVELOPE', str, 'RCPT TO');
                err.rejected = this._envelope.rejected;
                err.rejectedErrors = this._envelope.rejectedErrors;
                return callback(err);
            }
        } else if (this._envelope.rcptQueue.length) {
            curRecipient = this._envelope.rcptQueue.shift();
            this._recipientQueue.push(curRecipient);
            this._responseActions.push(str => {
                this._actionRCPT(str, callback);
            });
            this._sendCommand('RCPT TO:<' + curRecipient + '>' + this._getDsnRcptToArgs());
        }
    }

    /**
     * Handle response for a DATA command
     *
     * @param {String} str Message from the server
     */
    _actionDATA(str, callback) {
        // response should be 354 but according to this issue https://github.com/eleith/emailjs/issues/24
        // some servers might use 250 instead, so lets check for 2 or 3 as the first digit
        if (!/^[23]/.test(str)) {
            return callback(this._formatError('Data command failed', 'EENVELOPE', str, 'DATA'));
        }

        let response = {
            accepted: this._envelope.accepted,
            rejected: this._envelope.rejected
        };

        if (this._envelope.rejectedErrors.length) {
            response.rejectedErrors = this._envelope.rejectedErrors;
        }

        callback(null, response);
    }

    /**
     * Handle response for a DATA stream when using SMTP
     * We expect a single response that defines if the sending succeeded or failed
     *
     * @param {String} str Message from the server
     */
    _actionSMTPStream(str, callback) {
        if (Number(str.charAt(0)) !== 2) {
            // Message failed
            return callback(this._formatError('Message failed', 'EMESSAGE', str, 'DATA'));
        } else {
            // Message sent succesfully
            return callback(null, str);
        }
    }

    /**
     * Handle response for a DATA stream
     * We expect a separate response for every recipient. All recipients can either
     * succeed or fail separately
     *
     * @param {String} recipient The recipient this response applies to
     * @param {Boolean} final Is this the final recipient?
     * @param {String} str Message from the server
     */
    _actionLMTPStream(recipient, final, str, callback) {
        let err;
        if (Number(str.charAt(0)) !== 2) {
            // Message failed
            err = this._formatError('Message failed for recipient ' + recipient, 'EMESSAGE', str, 'DATA');
            err.recipient = recipient;
            this._envelope.rejected.push(recipient);
            this._envelope.rejectedErrors.push(err);
            for (let i = 0, len = this._envelope.accepted.length; i < len; i++) {
                if (this._envelope.accepted[i] === recipient) {
                    this._envelope.accepted.splice(i, 1);
                }
            }
        }
        if (final) {
            return callback(null, str);
        }
    }

    _handleXOauth2Token(isRetry, callback) {
        this._auth.oauth2.getToken(isRetry, (err, accessToken) => {
            if (err) {
                this.logger.info(
                    {
                        tnx: 'smtp',
                        username: this._auth.user,
                        action: 'authfail',
                        method: this._authMethod
                    },
                    'User %s failed to authenticate',
                    JSON.stringify(this._auth.user)
                );
                return callback(this._formatError(err, 'EAUTH', false, 'AUTH XOAUTH2'));
            }
            this._responseActions.push(str => {
                this._actionAUTHComplete(str, isRetry, callback);
            });
            this._sendCommand('AUTH XOAUTH2 ' + this._auth.oauth2.buildXOAuth2Token(accessToken));
        });
    }

    /**
     *
     * @param {string} command
     * @private
     */
    _isDestroyedMessage(command) {
        if (this._destroyed) {
            return 'Cannot ' + command + ' - smtp connection is already destroyed.';
        }

        if (this._socket) {
            if (this._socket.destroyed) {
                return 'Cannot ' + command + ' - smtp connection socket is already destroyed.';
            }

            if (!this._socket.writable) {
                return 'Cannot ' + command + ' - smtp connection socket is already half-closed.';
            }
        }
    }

    _getHostname() {
        // defaul hostname is machine hostname or [IP]
        let defaultHostname = os.hostname() || '';

        // ignore if not FQDN
        if (defaultHostname.indexOf('.') < 0) {
            defaultHostname = '[127.0.0.1]';
        }

        // IP should be enclosed in []
        if (defaultHostname.match(/^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/)) {
            defaultHostname = '[' + defaultHostname + ']';
        }

        return defaultHostname;
    }
}

module.exports = SMTPConnection;