After some help diagnosing why Exchange 2010 Enterprise stops responding to SMTP events.
I can't find a pattern to it. It doesn't appear to be an actual timeout, as the server responds immediately with the error. To reproduce it I telnet into the server on port 25 and issue a
EHLO. The server immediately replies with the 421:
421 4.4.1 Connection timed out
Once this starts happening I've found restarting the exchange box is the only reliable way to get it flowing again. Sometimes restarting the Transport service or the mailbox attendant service seems to fix it, but this could be coincidental as it often has no effect.