unable to receive external email
I've been running OS X Lion Server for about a year or more now on a mac mini. Mail service has been chugging away just fine until this morning when it stopped delivering emails from outside our domain to the users' inboxes. Email sent internally, from within the same domain, gets delivered just fine. And users can send emails both internally and externally. But users can receive mail only from within the same domain.
I have checked as many logs as I could think of and see nothing out of the ordinary. IMAP is running fine, postfix appears to be running without notable incident (though if I should be checking that more thoroughly, I'd take advice on how to do so - I've only checked the Mail log).
Senders do not get any notification of failure. And I do not see any errors in Server Admin.app (nor in Server.app). Perhaps I am not looking in the right places?
I even checked DNS, and the log there shows a ton of errors (connection refused), so I added a couple of new forwarded IP's just in case that might help. This may be a sign of my desperation, especially because the DNS should have little to do with email finding its way to my server. But I'm getting desperate.
Any advice and assistance is gratefully accepted.