Message hangs in outbox, won't transmit

I have had a cronic problem for 3 weeks where the Outlook2003 email messages just stack up in the outbox, but will not get released. It seems Outlook is trying but they hardly ever go. Sometimes when I get new mail, or after I reboot, they slip out okay. I have gone thru all the settings with ATT (my ISP). The connection is fine as Outlook express and the browser and Outlook 2003 receiving email works fine. I just can't seem to send reliably. Any suggestions?