We are having issues with messages stuck in the queue folder Messages with an unreachable destination. Also, some messages go through fine to an address one time, but another time the message will disappear. They are not in junk folders. I track the missing message and it says it was delivered to the Store. I have rebooted several times. Let me know if you need more info.
We are in the early stages of the migration - no email boxes have been brought over to 2010.