I posted about this in the GroupWise forum, but just to cover my bases,
I thought I'd ask about it in here from a different angle. I've got my
GWIA running on a bordermanager box. I don't want to deal with the
mail proxy, I've heard not so nice things about it.

Basically, the network address of the GWIA is set to a private IP. I
have the public IP, which our MX record points to, added as a secondary
ip. The link between the two are not in my NAT table. If I try to add
the link to the private nic of the border box and the public ip I
added, it says it already exists. I gather if they are on the same
box, the link must already exist.

Back to my question. Comcast's mail servers seem to want to do a
reverse DNS lookup before it will accept mail. That bombs out with an
error. Shouldn't, assuming that NAT link is working, DNS be handled
without any issues?