SBS Exchange Intermittently Not Delivering Email w/ no Undeliverable messages returned.

Windows Server 2003 Small Business Edition SP2.  Email appears to be arriving intermittently. They are not bouncing back from any IP or generating any undeliverable messages. People are saying they've sent messages, but on our side, they were never received. Some are though. It's very intermittent. I sent a couple emails to myself from my Gmail account and received all of them. Sent the messages from my Yahoo account and still have not received them. I am getting no undeliverable messages in return. I have searched through the IMF Archiver and none of the messages are in there. I have been checking it daily. This has been happening for about 4 or 5 days now. It's hard to say exactly when, as nothing has really changed. I applied SP2 per Microsoft's recommendations last night and the server.
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Check out message tracking and see if you can find any of these messages.  Also how is your internet connection?  If you are going up and down, mail sent would be queued on the remote server after the first try and then will try again, and again, until it times out and generates an NDR.  After 12 hours or more a delay notification is often sent to the sender.  So if you send the message right now, and havent received it, you probably wont get an NDR for a while if it is having problems contacting your server.

Do you have any kind of firewall hardware / software, any AV?

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jaysonfranklinAuthor Commented:
Where would 'message tracking' be?
It is in Exchange System Manager, under tools, then message tracking.  You will want to filter for the server, probably the only exchange server, the user, and the time that the message was sent.  

Has this been resolved for you?
jaysonfranklinAuthor Commented:
Yes. It was actually a user error...somehow there was a bogus client rule in outlook effecting about 5 people in our office. hmm. weird stuff. fixed now though. Thanks for your help. More than i can say for Microsoft....
Ha, good find.  Yeah, Microsoft isn't always helpful.
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