[Exim] Strange stuff

Hi,

My dad has an account with worldnet.att.net. He sent me an
email and it bounced, "relaying denied". I added worldnet.att.net
to relay_domains in exim.conf and everything is fine, but why
did it bounce from this particular domain?

I should mention that he has an account here which has a
.forward file in it, although I don't see why that should matter.

cu,

-Mark