This has happened to about 5 out of 100 users. Two of the five have 2007, and three have 2010, they all use exchange 2010. Sometimes when sending emails, the emails will be stuck in the outbox. By the time that they notice that something is wrong they have 6 or seven in there, and they usually will try to click on send/receive which freezes outlook.
I have tried disabling all add-ons and closing their archives, yet without archives and addons this still happens to them occasionally. I have also tried recreating profiles, reinstalling office 2010, and in the case of one user uninstalling office 2010 and installing 2007. Yet emails get stuck about every other week.