Unable to relay

A user has an emailadress from a different recipient policy (companyB.com)
than the normal users (companyA.com). Now if mail arrives at the exchange
server it replies
:
Sent <<< RCPT TO:(E-Mail Removed)

Received >>> 550 5.7.1 unable to relay for (E-Mail Removed)


CompanyB.com is a recipient policy. Internal mail traffic to the user in
question is working fine. Both when we send mail to the user name "user"
and when we use the full mailadress (E-Mail Removed) as destination.

I've checked relaying settings, but I don't think this is a relaying issue
since the server which responds "unable to relay" is the server where the
mailbox is located.

Furthermore I have a second Exch2k3, also running on win2k DC(Which is also
GC), in the same site. If I connect with telnet to SMTP and try to deliver
the mail it gets delivered throug a routing group connector. If I telnet the
first server I get unable to relay.
I've checked SMTP virtual server properties and they are identical on both
machines. Are there other settings for relaying, prohibiting relaying?

Any pointers anyone?

regards, tnx in advance.

david