how about..
The following recipient(s) could not be reached:
(editted for clarity ;-)
'ehlocon at microsoft dot com' on 2/8/2006 9:39 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.
<smtp.myemailserver.com #5.1.1 smtp;550 5.1.1 User unknown>