I also get this in my smtp logs:

Sep 01 10:00:01 2004 (8010) <mailman.0.1094032801.6963.mailman@chemtool.be> smtp for 1 recips, completed in 0.031 seconds Sep 01 10:00:01 2004 (8010) All recipients refused: (-2, 'Name or service not known')

digests.mbox is where posts are collected for eventual inclusion in the digest. It has nothing to do with why the subscribers aren't receiving mail unless they're all subscribed in digest rather than message mode.

Visit the FAQ wizzard at http://www.python.org/cgi-bin/faqw-mm.py and see article 3.14 for some other things to check.