EMail Sent but not Received

I have a Win XP Pro computer that sends email from outlook or
mozilla thunderbird, moves the email to the Sent folder, but the
email never gets to the recipient. Using Internet Mail Only, no MS
Exchange. Other cpus on the same network, with the same ISP
host, same domain, have no problem.

Repaired the .pst files, installed and uninstalled Outlook 200/
2002/2003/Mozilla Thunderbird. Same result.Possible clue-
Outlook express was running on the same machine with settings
for a different user. Email stopped sending at about that time.
TIA for any insight into this.