I ran into this error recently while trying to migrate from Exchange 2003 to Exchange 2010. I had my Exchange 2003 and Exchange 2010 mailbox servers in coexistence, and had successfully migrated a pilot group of users to the new server. Mail flow was still be directed through the Exchange 2003 server, although I did have a Send Connector on the 2010 server.
Users on both the 2003 and 2010 servers could send and receive mail externally. Users on the 2003 server could send messages to users on the 2010 server. However, users on the 2010 server could not send message back to the 2003 server. The queues would just fill up and 2010 users would receive a delivered delayed message. The error on the message queue was the one mentioned above.
The fix was two-part for me. [Read more…] about 451 5.7.3 Cannot achieve Exchange Server authentication