Some mail Staying in outbox Connectted to Exchange 2010

Hi

My users are having problems with email (lets say 1 out of 50) staying in the outbox of their Outlook 2007 client and just won't send. I have been able to get their email sent by opening a new blank email, Copying the contents of the email in the outbox then sending it and it will send. But the email in the outbox just sits until it is deleted. Any ideas on why this might be happening?

Thanks