Exchange 2010 SMTP Error 451 Transaction failed

There is nothing as much annoying as having useless error message. And when you combine this to the poor log entries I’ve found on both Event Log and the Connector text-based log file, you end up so upset and have no clue what the f**k is going on?

The mail system works fine except with sending & receiving from outside (over the internet). At the first, we thought it’s a firewall issue, so we’ve reviewed all the rules, and even we restarted the firewall. But when digging some more, the SMTP service is responding fine to connection from outside. I’ve connected from out of the corporate network (a remote server) and used the simplest method, “telnet ourmailservername 25”.

The command opened normally, and I’ve got the welcome message from the Exchange ESMTP service. OK!!! Let’s continue, helo / ehlo, mail, rcpt commands all works just fine till you start the data. What happened? after accepting data and once I send the <cr>.<cr> command to end the message and instead of getting message ID ********* have been queued for delivery, I got the above stupid message “451 Transaction failed”!!!!!

Three hours of investigation and monitoring went for zero value. Until the networking team decided to accept part of the responsibility and rebooted the internet router ( something between the firewall and the ISP link). Yep, that solved the problem on spot.

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