Tons of Undeliverable Messages - need your help

I have a user in our network that is getting lots, maybe hundreds of Undeliverable messages. Many have subjects about Rolexes. I first thought this was just someone use this users email address as the from. Then after looking at the emails a little closer, they do not have headers and when blocking emails with "rolex" in the subject at the email server did not stop them, I began to believe these non-deliverabe are being generated internally. The emails are usually from "System Administrator" and have a subject line typical of a SPAM email. The content is usually something like


Your message did not reach some or all of the intended recipients.

Subject: WGb Women Watches / Rolex / Cartier / for sale !!
Sent: 8/6/2006 2:46 PM

The following recipient(s) cannot be reached:

xxxxxx@firstcb.com on 8/6/2006 1:47 PM
The e-mail account does not exist at the organization this message was sent to. Check the e-mail address, or contact the recipient directly to find out the correct address.
<fcbnt2.firstcb.com #5.1.1>


Norton AV scans have not yeilded anything. MS Defender is running on this persons machine.

Any ideas as to what might be causing this?