Server responded: 503 AUTH command used when not advertised\r\n

smtpsmtp-auth

I recently migrated from one server where my mail settings worked without a problem to a new server which now results in the following error:

Server responded: 503 AUTH command used when not advertised\r\n.

The full SMTP log is:

19:16:12 [MailBee SMTP v. 5.3.0.47. Registered version]
19:16:12 [Connecting to server mail.example.co.uk at port 25 using default domain]
19:16:12 [Domain specified in FromAddr, example.co.uk used]
19:16:12 [Entering ESMTP authentication mode]
19:16:12 [Sending ehlo]
19:16:12 [Using PLAIN ESMTP authentication]
19:16:12 [Error: Negative or void server response]
19:16:12 [Server responded: 503 AUTH command used when not advertised\r\n]
19:16:12 [Possible error reason: PLAIN authentication type (auth plain command) is not supported by this server]
19:16:12 [Error: Connection failure]
19:16:12 [Error: Not connected]

Please could someone point me in the right direction as to why this is failing.

Best Answer

You cannot use AUTH when the server don't allow you to. The server would say something like

250-AUTH PLAIN LOGIN

if you would be allowed to. In your comment it's also visible that the server haven't advertised SMTP auth so using it violates protocol.

The most probable reason is security: the server announces STARTTLS and most probably expects you to use TLS (encrypted/secure connection) before you try to use a non-encypted authentication, basically protecting you from negligent behaviour.

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