I have set up a Postfix Mailserver, outgoing mail is being sent through a smarthost/relayhost which requires authentification. That works great, internal clients can send to foreign recipients though this relayhost.

However, when an external mail for a local, non-existent user arrives at the server, postfix tries to send a non-delivery notification to the sender. This mail is also sent through the relayhost obviously, but it fails with error 554 5.7.1 : Relay access denied

This gets logged to the mail.log:

Nov  9 10:26:42 mail postfix/local[5051]: 6568CC1383: to=<[email protected]>, relay=local, delay=0.13, delays=0.02/0.02/0/0.09, dsn=5.1.1, status=bounced (unknown user: "test")
Nov  9 10:26:42 mail postfix/cleanup[5045]: 85DF9BFECD: message-id=<[email protected]>
Nov  9 10:26:42 mail postfix/qmgr[4912]: 85DF9BFECD: from=<>, size=3066, nrcpt=1 (queue active)
Nov  9 10:26:42 mail postfix/bounce[5052]: 6568CC1383: sender non-delivery notification: 85DF9BFECD
Nov  9 10:26:42 mail postfix/qmgr[4912]: 6568CC1383: removed
Nov  9 10:26:43 mail postfix/smtp[5053]: 85DF9BFECD: to=<[email protected]>, relay=mail.provider.com[168.84.25.111]:587, delay=0.48, delays=0.02/0.01/0.26/0.18, dsn=5.7.1, status=bounced (host mail.provider.com[168.84.25.111] said: 554 5.7.1 <[email protected]>: Relay access denied (in reply to RCPT TO command))
Nov  9 10:26:43 mail postfix/qmgr[4912]: 85DF9BFECD: removed 

According to this error, I suppose that postfix does not login at the relayhost when sending those bounces. Why? Normal outgoing mail works just fine.

This is how my main.cf looks like: http://pastebin.com/Uu1Dryxy And of course /etc/postfix/sasl_password contains the correct credentials for the relayhost.

Thanks in advance!