Outlook not behaving

I have a prolblem trying to send email from only one client on my network. It is a Win XP Pro OS with Win XP Pro Outook. It receives email fine. But will not send (the emails actually show up in the “sent” folder) What I have done:

– checked and rechecked the outgoing mail server
name.
– Put the end-users info (username, password, mail server names) on another computer and everything worked fine.

So this proves that something on the first desktop is not working properly. I really don’t want to format just to fix this problem. Has anyone else seen this?