I have been battling the above frustrating error for a few months now, and never actually found a good workaround until today. I started having the issue when I was running Outlook 2010 Beta, so I just shrugged the issue off as an Outlook beta problem. I’m now running the proper RTM version of Outlook, and the issue still exists. I have my primary account setup with Exchange on our Exchange 2010 server, and a handful of POP3/SMTP accounts setup that pull mail down into my Exchange box, and give me appropriate Send As rights for other accounts I need to be able to send from, that exist outside my organization.
Every now and again, I would notice my Outbox had stuck mail in it… pretty unusual for an Exchange box. The only messages that would stick are ones destined out a POP/SMTP account, with a file attachment. After chasing down permission issues, virus scanning, relay permissions, server issues, etc. I was pretty stumped.
The whole problem here is a bug in Exchange 2010, it was present in the Beta version, and is still present as of RTM Update Rollup 3 (will be trying again with Update Rollup 4 when we can appropriately test it). The issue only exists if you do not run cached Exchange mode. If you wish to leave cached mode off (as I do), you can work around the issue if you set your POP3 accounts to deliver mail to an alternate data file, a simple PST. For my purposes, this is fine, I just changed my alternate accounts to forward to my Exchange primary address, so e-mail still makes it to my inbox, but sent mail simply sits under the local data PST file. Once Microsoft gets around to fixing the issue, I’ll simply merge the sent folders back into my Exchange Mailbox, and put this issue behind me.
I’m glad I found this useful thread that helped shine some light on the issue. I was approaching the problem from my logical perspective, and that was between my Outlook and the SMTP server… why would the dropping of the mail into my Exchange Mailbox Sent Items have anything to do with this? I’m still not pleased that this bug exists, but at least there’s a workaround. This issue only happened when I went to Outlook 2010, because that was the point in which I swapped away from Cached mode.