On Exchange 2010 we had a strange error show. Mail was working the previous day and strange enough mailbox moves were working.

Here is the error:

• 451 4.4.0 Primary target IP address responded with: “451 5.7.3 Cannot achieve Exchange Server authentication.” Attempted failover to alternate host , but that did not succeed. Either there are no alternative hosts, or delivery failed to all alternative hosts.

Solution:

First thing you check with that kind of error is if exchange server authentication is enabled on your default receive connector. BUT, it was working the day before and nothing changed.

If you are running CISCO ASA Firewalls then you might want to tweak or disable ESMTP Inspection. In this case it was disabled and mail flow resumed as normal.

Hope it helps.