Approximately a few days ago, users sending mail to some sites started getting their mail kicked back to them. Looking at the SMTP logging in Event Viewer, we were receiving an error 451. Looking at the queue properties for the non-deliverable mail,the error that we were receiving was using generic terms:
[451 4.1.8
Doing an nslookup for the mx record to the recipient domains was successful, and an nslookup using the recipients server to do a lookup on our mx record was successful, ping and tracert were also successful, and telnet sessions to the remote mail servers also worked, so there is definately communication back and forth between our respective email servers. Our Exchange server is on NT 4.0 with service pack 6, and Exchange has service pack 4. If anyone has any idea what the cause of this error is, could you please help! I did find the same error reported by an nscloss in this Q&A, but no answer to the problem. I have been searching everywhere!
