NW6.5 sp7
GW 8 hp1

I upgraded to GW8 about a month ago. Everything has gone without a hitch. I
had a mail relay in place that GW relayed all outbound email to. Again,
there have been no issues with this set up.

Recently, the relay was removed, MX record changed from the mail relay to
the GW server. I made sure that there was a reverse DNS lookup for the new
MX record with our ISP.

Now, out of the all of the emails that are sent each day, I have a handful
of them that keep failing with a 501 5.5.4 Invalid address on the server. I
am also getting a couple of 501 Invalid Domain Name errors on the server
also. They are always to the same domain. My mail server points to my
internal dns server that forwards all external requests to my ISP's dns

Any idea what I can do to fix the problem?