Outlook 2003 – Email stuck in Outbox – Intermittently

From time to time, a message or two will get “stuck” in the Outlook 2003/Outbox.
I have to perform a couple of things to get the emai to “go”.
1) I have the user shutdown Outlook and restart.
2) I can move the message to the Drafts folders and then back to Outbox.
3) I have the user move message to Draft folder & shutdown/restart Outlook.

All service packs for O/S [XP-Pro] and Outlook 2003 are up to date.
ISP POP & SMTP servers are working fine.

Hopefully, someone can give me a reason as to why this occurs intermittently. If there is a “patch” to fix, that would be a plus. Thanks, Ron Maagero