[Mailman-Users] Senmail Service unavailable bounce!!

[Simon, imagine media]

hello all,

I am getting "dsn=5.0.0, stat=Service unavailable" messages from sendmail
for some addresses in my list! Mailman is working correctly for other
addresses (see below):

Feb 11 00:05:07 localhost sendmail[13628]: g1B052913626:
to=<munchpup at hotmail.com>,<katrinakirk at hotmail.com>, delay=00:00:05,
xdelay=00:00:05, mailer=esmtp, pri=199930, relay=mx04.hotmail.com.
[64.4.56.135], dsn=2.0.0, stat=Sent (Requested mail action okay, completed)


But bounces other addresses:

Feb 11 00:05:08 localhost sendmail[13628]: g1B052913626:
to=<bigfish at mac.com>, delay=00:00:06, xdelay=00:00:01, mailer=esmtp,
pri=199930, relay=smtp-mx.mac.com. [204.179.120.49], dsn=5.0.0, stat=Service
unavailable
Feb 11 00:05:08 localhost sendmail[13628]: g1B052913626: g1B058913628: DSN:
Service unavailable

I can successfully send to the bounced address (both as root and mailman
users) via elm from the same box:

elm -s "Testing Webmaster email" bigfish at mac.com < /dev/null

Feb 11 09:49:13 localhost sendmail[21942]: g1B9nDi21942: from=root,
size=223, class=0, nrcpts=1,
msgid=<200202110949.g1B9nDi21942 at ns0.imagineinternet.co.uk>,
relay=root at localhost
Feb 11 09:49:17 localhost sendmail[21945]: g1B9nDi21942: to=bigfish at mac.com,
ctladdr=root (0/0), delay=00:00:04, xdelay=00:00:04, mailer=esmtp,
pri=30223, relay=smtp-mx.mac.com. [204.179.120.49], dsn=2.0.0, stat=Sent
(g1B9plV7021606 Message accepted for delivery)


I need a way of finding out why Mailman cant send to these addresses.

Simon