Follow up of#14495

If we don't callmessage.Close() once the method exits, we could leak resources.
This restores thedefer, but conditionalizes it such that it will only callClose() 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