I had a notice from Mailman for one of my lists that there was a message that required administrative attention (i.e. it was spam). The mailman web-interface provides a facility for treating email, however when I went to discard the message it wouldn’t go away. I tried a heap of things and it took a long time to figure out, but eventually I noticed that in the mailman web page the domain name it was using was “intranet.blackbrick.com” whereas that should have been “www.intranet.blackbrick.com”. So I did a little searching about how to configure a list’s base URL and discovered this, which fixed my issue:

/usr/lib/mailman/bin/withlist -l -r fix_url support \
  --urlhost=www.intranet.blackbrick.com

Where ‘support’ above is ‘list name’.