When a message is forwarded or released, Mailpit introduces itself as
`localhost` to the upstream server. This happens because `net/smtp` forces the
value to be `localhost` if `client.Hello` is not called. This is explicitly
documented at https://pkg.go.dev/net/smtp#Client.Hello
Therefore, both `internal/smtpd/relay.go` (`createRelaySMTPClient`) and
`internal/smtpd/forward.go` (`createForwardingSMTPClient`) should either call
`client.Hello(os.Hostname())` or create a config (perhaps `config.HeloHostname`)
and use `client.Hello()` with that value immediately before returning from
either of those functions. (The HELO/EHLO command comes after TLS negotiation
but before any other SMTP commands.) This commit does the former.
Without this feature, Mailpit cannot be used in combination with Google
Workspace's SMTP Relay functionality, as it rejects any connection that
identifies itself as `localhost`.
Relates to #146
- Updated error handling to use the error return value for resource closures in tests and functions, ensuring proper error reporting.
- Replaced direct calls to `Close()` with deferred functions that handle errors gracefully.
- Improved readability by using `strings.ReplaceAll` instead of `strings.Replace` for string manipulation.
- Enhanced network connection handling by adding default cases for unsupported network types.
- Updated HTTP response handling to use the appropriate status codes and error messages.
- Removed unused variables and commented-out code to clean up the codebase.
