Make SMTP auth reply code checks work properly.

Currently only `334` and `503` can be returned by the `smtpSend()` call for the `AUTH LOGIN` command, so the checks for `== 504` and `!= 503` will never be reached, and in case the server would respond with a `503` it would not cause an exception to be thrown as there are not further checks after `!= 503`.

As far as I understand, a server should either return `500` or `502` depending on whether the `AUTH` command (respectively commands in general) is not recognized or not implemented, however I've seen servers issuing `501`s in such a case too, but I'm not quite sure whether this is violating the RFC, so I didn't made use of that one.

Also the exceptions for the username/password will never be thrown, as `_smtpSend()` already triggers one in case the reply code to check is not being matched.