POP Email Clients outbound emails to Exchange 2003 Server are disappearing

This is a private exchange server 2003. Most users are connected through Outlook 2010 Exchange protocol. A few users connect via pop setting the Exchange Server as the pop server and the smtp server. 2 days ago the outbound emails stopped flowing. The Outlook client says they are sent, but clients never receive them.
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Do you see the emails in the Exchange queue?
Can you do a trace by using the Exchange message tracker?
Do you see the email in OWA?

If you see the email on the server and OWA but not on Outlook then the problem is with Outlook not displaying propery.  Take it off cached mode to see if the sent items get updated.
rhillsnapAuthor Commented:
The same scenario is occuring on all 4 users that are setup this way.
No, the email is not showing up in OWA Sent Items.
How about when you using the Message Tracking Center to track a message in Exchange Server?  Do you see any trace of the missing messages at all if they were passed to Exchange?
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rhillsnapAuthor Commented:
I looked through the log file and it does show traces of the messages in Exchange
rhillsnapAuthor Commented:
These 4 users are in England and we are in Dallas. That is why they pop in to retrieve email. But I set the same scenario up here locally and am getting the same results.
Ok, so Exchange did received the message it's just not in Outlook.  Have you tried taking Outlook off the cached mode (uncheck it).
rhillsnapAuthor Commented:
I do have 2 smtp virtual servers setup. One is on port 25 and the other on 465. The  4 in England are setup on 465 and I set mine up to port 25. Neither work.
rhillsnapAuthor Commented:
I dont think there is Cached Mode for POP setup
rhillsnapAuthor Commented:
I just sent another message and it shows up in the Exchange logs, but does not get delivered and it doesnt show up in OWA. If I send the same message from OWA it gets delivered instantly and works perfectly. It is just when you send from client using the POP Configuration on all 4 machines.
rhillsnapAuthor Commented:
I am getting event log 953 and System Manager says the Exchange Routing Engine is stopped. Dont know if these are  routine or part of my issue.
rhillsnapAuthor Commented:
Could an expired certificate cause this issue?
rhillsnapAuthor Commented:
It turns out my outbound spam filter was classifying those emails as unlicensed and quarantining them without any notification. Very frustrated with Proofpoint at the moment. It tooks hours researching this and 2 phone calls to them to finally resolve.

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Hi rhillsnap, sorry for the late reply.  Glad you got it sorted out. Cheers!
rhillsnapAuthor Commented:
They made a couple of valid attempts to help, however I ended up solving it myself. It was a very tricky issue for anyone to resolve remotely.
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