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Emails queuing for one specific user

Messages from one specific user in our Exchange 2010 environment are getting held up in Queue.
Internally he can email to people within the company without issues.
When he send to external users the messages hang in queue.
We get a 4.4.7 Message Delayed error.

Eventually he gets an NDR with  "#550 4.4.7 QUEUE.Expired; message expired ##" message.

We use for our smarthost.

We have a couple hundred users that are fine. It is just this one user's EXTERNAL messages are the only ones that are getting stuck.

I restarted the Exchange Transport services. No fix. All other email flows for other users so I suspect that the smart hosts are working fine.

How should I troubleshoot? I'm baffled how this only effect this one user.
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Note: I have checked and our domain is not on a blacklist.
Used for this test.
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Will Szymkowski
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The queuing happens for multiple external domains. Most of the messages are simple replies to messages sent to him.
I'll check OWA. We have two mail servers at the edge so I'll ask him to try to send one message from each.
Telnet pointed us in the right direction. Possible routing/firewall issue on one mail server. We can't telnet out to any other mail server from that box.
Turns out that the user in question is the ONLY user on that mail server database. Moved him to the other server and we will troubleshoot that bad Exchange box or simply take it out of the loop.
@Will's advice to telnet from that server led us to the fix.
After a telnet failure to all external servers from that Exchange box we know there is an issue there. Moved the user to a different mail server/database and his mail works fine. Now we will address the issues on the bad Exchange box. I suspect it was a side effect from some networking issues we had earlier in the week and the email flow issue was just reported to us. It has probably been going on for a couple of days.

Well done.