Recently I migrated my user mailboxes from Exchange 2007 to Exchange 2013. All servers are located on site as well as the affected users mailboxes. Recently I've been having users emails hanging in their outbox for no apparent reason. The users are on Windows XP SP3 and run Office 2007 SP3. This issue pops up and sometimes will go away on its own after a few hours. We have tried new mail profiles, reinstall office, and repair office. Nothing works. Has anyone seen this issue before? Nothing in the event logs point to any errors.