We're running outlook 2013 + Exchange 2010 SP3.
I have a user that twice now has called helpdesk to say his email is stuck in his outbox. His client was in cached mode but connected to the exchange server. The first time, after trying a manual send/receive, I exited outlook and very quickly saw the outbox empty of the 9 items that were there. I went over to sent items expecting to see them there and they weren't. The recipients never received the emails.
Today, the same thing happened (it's 2 weeks later and he hasn't had the problem since) but this time we saved everything before exiting outlook. When outlook started again, the messages in outbox were deleted. I did message tracking on the transport servers and they never made even made it to the hub transport. However, because I had a record of the precise emails he was responding to this time, I search for them and they actually show that he has replied at X:XX time, but they were never sent.
I think this has something to do with cached mode so I've switched his client to online mode in the meantime but I was wondering if anyone has seen anything like this. It's the first time I've ever seen outlook (Any version) actually delete email.