|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.