fix: restore closing SMTP message on method exit

[dannykopping]

Follow up of #14495

If we don't call message.Close() once the method exits, we could leak resources.
This restores the defer, but conditionalizes it such that it will only call Close() if it hasn't already been called. If we don't do this, the SMTP lib will send a subsequent line to the server after an error response, which is invalid. It's not a trainsmash, but it's better to be defensive.

.                                            <--- initial Close()
501 5.5.4 Rejected!
.                                            <--- deferred Close()
501 5.5.2 Bad command