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 Error Message Summary

Error Code/Message


What to do

421-failure in DNS look-up.
There seems to be a failure in your IP reverse DNS lookup. Your IP address must have a valid reverse DNS entry to send mail to the AT&T domain. Please correct the error or contact your service provider for assistance before trying again. The system administrator should consult

450 busy--Please try again later

The sending system has requested more resources than are available at the moment.

The sending system should re-try.

521 <> is blocked

The sending system has been identified as a source of spam.

Please see for more infomation.

550 [suspend] Mailbox currently suspended--Please contact correspondent directly.

The account of the BellSouth customer is suspended.

Contact the customer directly.

550 [permfail] requires valid sender domain

We were unable to verify the identity of the sending system.

Administrator of sending system should enter correct information in DNS (domain name system)

550 unauthorized interface for <>

The sending system is not recognized as a valid source of e-mail.

Correspondents should use registered mail systems provided by a major ISP or hosting service.

550 <> blocked by ldap:ou=rblmx, dc=bellsouth, dc=net

The sending system exhibited behavior associated with spammers and was blocked.

After taking steps to mitigate their spam or virus problems, the adminstrators of affected systems can send unblock requests from



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