Outlook Express - possible virus?

[ComputerNovice]

I'm no desktop expert, so I'm hoping that someone can shed light on this, if possible.

My cousin runs a small business, and he uses Outlook Express as his email client. Whenever he selects the send/recieve button to send an email, his computer automatically sends multiple emails (between 10 to 15 each) to his customers, some emails of which have already been sent out. It also appears that there is a recipient from his address book that is receiving emails, who shouldn't be getting them.

Nobody really maintains his computers, so I went to the Microsoft website to see if his updates were current, which they are. I alo ran the Microsoft Malicious Software Checker, which came out negative for any malicious code. Then I went to the Avast website, and ran one of their free utilities to check for a virus on his computer, which also came out negative. So if there is a virus on the computer, it certainly doesn't appear to be getting picked up or detected by either utility.

Has anybody experienced this before?

[ComputerNovice]

For future reference, I just want to let anyone know that the problem was due to the scan of outgoing email through Zone Alarm's firewall. I simply turned the option off (while still allowing Zone Alarm to scan incoming email), and then deleted all messages sitting in the outbox. Fixed it right up.

I've read that this can also be a problem with other virus software that scans outgoing mail.