Sounds like you do not have inverse-DNS setup, which is the answer to your
question.
Not to mention a source of refused connections at many ISP's, which is a
basic security check (or double check, depending) and one of my pet peeves
with other systems.
------------
Jeff Mountin
sysop@mixcom.com
MIX Communications
Serving the Internet since 1990
Sure my business card says "Senior Network Administrator"
They still make me do just about anything.