The first I heard about the problems we're experiencing, was when somebody contacted me about emails that were not getting through. People were receiving a "Delivery Failure Report" that said "Delivery to the following recipients has been delayed".
The first thing I did was have a look at the SMTP queue on our server and to my suprise there was five days worth of email in the queue that hadn't been sent.
I also checked the event viewer and found 1000's of SMTPSVC warnings that read:
Message delivery to the host '<ip address>' failed while delivering to the remote domain '<domain name>' for the following reason: The remote server did not respond to a connection attempt.
Any idea what the problem could be?