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 smtp-relay.gmail.com 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.
Used MXToolbox.com for this test.