[Mailman-Users] Some Posts not getting through - Troubleshooting tips?

[Bruce Best (CRO)]

I have recently installed mailman, and have currently set up a list with two
subscribers to test it out.

Mailman is set up and is working fine. However, one of the list members
never gets any posts. There are no bounces, no indication  that anything has
gone wrong. Just no posts.

The server/MTA (exim4) that is running mailman can send email directly to
the problem email address, and it gets through to the user no problem. It is
only when the email comes from the mailman list that it gets silently
dropped.

The only potentially useful log I can see is the "connection timed out"
lines in the exim4 logs, which are the only thing showing that exim is
trying to send the email and it isn't getting through:
 
2005-02-10 11:28:28 1CzGvq-00060p-VL == user at example.org R=dnslookup
T=remote_smtp defer (110): Connection timed out
2005-02-10 11:28:36 1CzH58-00061V-DZ mailserver.example.org [0.0.0.0]:
Connection timed out

(email address, mailserver and ip changed in above)

There are no bounces or other errors in the mailman logs. 

I am guessing that my list is being caught in a spam filter; it sounds like
this entry from the FAQ;
http://www.python.org/cgi-bin/faqw-mm.py?req=show&file=faq03.042.htp.
However, given that regular emails are getting through, and given that
mailman is running on a recently set up domain/ip address which has never
done anything to hurt anyone before, this seems unlikely.

Any other thoughts on how to figure out what exactly is happening to these
posts? 

Mailman 2.1.5 & Exim4 running on Debian Testing, installed via apt-get
(mailman_2.1.5-5_i386.deb)

B.