[Exim] 550 User unknown while rejection

Hi!

Some host on the net tries to deliver mail to a user on our server. The
message is rejected due to ORBS check.

The session transcript is:

>>> RCPT To:<[user@domain]>
<<< 550-Open realy - see ...[ORBS]
<<< 550 mail from [IP] rejected: administrative prohibition

Fairly common I think.
But just the next line says

550 <[user@domain]>... User unknown

Why that? We have that address on the system.

Daniel