Disappearing Sent MobileMe email

A client of mine is using his mobileme email in Mail on his MacBook running snow leopard.

He has a lot of emails just disappear when he pushes the send button. The email won't be in his sent box, or his trash, but sometimes the recipient still gets it. if I check online in his sent messages folder, it isn't there either. What could cause the email to behave this way? I will probably delete the account and re-enter it into the Mail app, but this seems to be an unusual problem. It is a recurring issue as well, not a one time thing.

Any ideas?

Thanks

PS - I posted this link in a different place, but it didn't get many looks. I think this new discussion interface is still a bit confusing, I like the new functionality but it is harder to search for what you want I think.