Enhanced the documentation by adding detailed comments for each SMTP authentication type, specifying their references to RFC documents. Corrected comments for error variables to match the corresponding authentication schemas.
Introduced the SMTPAuthNoAuth mechanism in the auth.go file meant for instances where authentication is not required or supported. Adjusted tests in client_test.go to account for this new mechanism. Minor order modifications were also made to the list of supported SMTP AUTH types and respective tests for better consistency.
Closes#176
# SUMMARY
* Bad licenses:
* Deprecated licenses:
* Licenses without file extension:
* Missing licenses:
* Unused licenses:
* Used licenses: CC0-1.0, MIT
* Read errors: 0
* Files with copyright information: 45 / 45
* Files with license information: 45 / 45
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