|I have no idea what could cause a server to tell you that a user is unknown |when he actually exists. I suppose there are probably all kinds of reasons |for it, mostly due to configuration errors and other system problems on |the user's host. It's been my experience that when the mail server MTA indicates that a user is not know and it is not prime time e.g., mid-nite at the server site, then the authentication system upon which the MTA is depending upon is temporarily FUBARed or unavailable e.g., it is being backed-up and not responding to inquiries. YMMV.