One of my users is trying to send email and gets a 5.1.1 NDR (the recipient is a user in my domain, but is not email enabled). I can send email to this recipient without any problems. We operate in the same domain, off the same server, etc. The only real difference is that I use Outlook 2002 and he uses Outlook 2000. The categorizer tells me that it receives the message from my user and then generates an NDR, but from me it delivers it. Any ideas?