Misc. fixes (#267)

* Misc. fixes

- Do not expose 'username' to a potential recipient.
- Make 'from' mandatory, because there is no fallback anymore.
- Validate 'from' to make shure, it contains an email address.
- Avoid namespace pollution: Use let instead of var.

* Suggested changes from codereview

README.md

@@ -39,10 +39,11 @@ Some features:
     password: ${{secrets.MAIL_PASSWORD}}
     # Required mail subject:
     subject: Github Actions job result
-    # Required recipients' addresses:
+    # Optional recipients' addresses:
     to: obiwan@example.com,yoda@example.com
-    # Required sender full name (address can be skipped):
-    from: Luke Skywalker # <user@example.com>
+    # Required sender (Either: "Plain Simple Name <user@doma.in>" or just "user@doma.in" (without the <>))
+    # Important: '<' and '>' are special chars in yaml. Therefore this string should be quoted
+    from: 'Luke Skywalker <user@example.com>'
     # Optional plain body:
     body: Build job of ${{github.repository}} completed successfully!
     # Optional HTML body read from file: