I just noticed that Mailman stopped working about 6 weeks ago. I have a (dv) server at MediaTemple and it runs Plesk 9.x. This has been an issue for a while now, where it randomly stops working. If you have the same issues with Mailman under Plesk, here’s a fix for you. Run the following command:

$ /usr/lib/plesk-9.0/mailman_conf_init  /usr/lib/mailman/Mailman/mm_cfg.py  qmail

This will force a reconfiguration of Mailman for qmail. After that, restart qmail and Mailman

$ service restart qmail

$ service restart mailman

Found and modified via Plesk forums