Outlook Outbox

I have only one user this is happening to. But we are on the ancient MS Mail, and use a seperate internet mail collector as a server for Internet Mail. On one system only, when he sends mail, inter or intra, it stays in his outbox. The recipient recieved the mail, but it is in outbox. The user swears they did not change any settings.

Where can I look to fix this?

Thank you.