After upgrading to MS Outlook 2010 some users started reporting an intermittent issue:
Occasionally messages get stuck in outbox even if Outlook is connected to the Exchange Server and shows that all folders are up to date. When a user clicks on Send Receive, Outlook hangs or crashes. After Outlook is restarted all messages from outbox are gone.
Sometimes Outlook would hang simply typing a message. Outlook AutoSave feature was also working sporadically, sometimes not saving messages even after 10-20 minutes when it was set to autosave to drafts every 3 min.
All this resulted in users losing some carefully crafted emails causing lots of frustration.

A quick Google search revealed number of people complaining about the same issue on MS forums, but no real solution.

First of all, we tried the following:

• Disable all add-ins
• Recreate new profile
• Reinstall MS Office

Nothing helped.

Then we checked Outlook versions within a company and it appeared that all affected users were using version 14.0.5123.5000 when the rest of the company was on 14.0.6023.1000, which is actually Outlook 2010 SP1.
Installing MS Office 2010 Service Pack 1 definitely helped, but the issue wasn’t completely resolved. 

Eventually we tracked that the problem was caused by users storing .pst files in network locations. Mostly it affected users who were moving with their laptops around a lot (connecting and disconnecting from the network).

MS Exchange 2007
MS Outlook 2010