MailMan with postfix (relay access denied)

I'm using trying to implement MailMan with my server. However, when
attempting to add a user outside my domain I get the
following error:

atom postfix/smtpd[16750]: D42B593D921: reject: RCPT from localh
ost[127.0.0.1]: 554 <molmod@sbcglobal.net>: Recipient address rejected:
Relay ac
cess denied; from=<mmodlist-bounces@atom.chem.iupui.edu>
to=<molmod@sbcglobal.ne
t> proto=ESMTP helo=<atom.chem.iupui.edu>

I can send mail via my server to this person without fail:

atom postfix/smtp[17611]: 5254393E50C: to=<molmod@sbcglobal.net>
, relay=sbcmx3.prodigy.net[207.115.57.15], delay=2, status=sent (250
2.0.0 i4RHe
QLE032725 Message accepted for delivery)

Why will my server accept in one case and not the other?


Thanks,

Kelsey

Kelsey Forsythe, PhD
Director, Computational Molecular Science Facility
IUPUI Chemistry
LD 320
402 North Blackford St.
Indianapolis, IN 46202
Ph: 317-278-2202
Fax: 317-274-4701