I'm getting this error on an sbs 2003 with exchange 2003, all latest SP's.
550 5.7.1 Unable to relay for firstname.lastname@example.org
Users are receiving this error when sending to SOME domain, not all.
There was an open relay on this server which I fixed.
The exchange settings are correct: relay > relay for only the list below > local ip of the server.
The domain is not blacklisted anymore.
Any idea why some users are getting this msg?