I've seen this behavior on a couple of our workstations and the problem recurs occasionally.
A user with Windows XP SP2 and Outlook 2003 will accumulate a number of messages in their Outbox. There is apparently nothing wrong with the outgoing messages except Outlook simply fails to successfully send them. No error message is given. Upon closing and re-opening Outlook the messages "magically" go out. Does anyone have any idea what could be causing this?
These Outlook installs use both a POP3/SMTP server running Courier on Linux and a Microsoft Exchange server for collecting and sending mail. The Linux server is the default account.