Last week I had a customer who started having problems sending emails to one particular recipient.
The user was getting bounce mails with the error
> #550 4.4.7 QUEUE.Expired; message expired ##
Which typically would mean that the sending server has given up.
Protocol-logs show that the server initiates a connection, but that’s it, no further logging. It never gets as far as trying to send an email.
The queue says the error message is Helo command rejected: Host not found.
Which would suggest that the receiving server doesn’t accept the HELO/EHLO command, because it’s running a reverse lookup to verify senders identity.
Now, why is it doing this? Because a pretty basic setting has been skipped.
Under the servers send connector, this field was blank:
Which means it didn’t identify itself properly. Set it as the servers external address and thar she blew!