After seeing a question on linux-quebec mailing list about this, a quick search turned up several entries. It’s intereseting how encryption tools also provide authentication means that can be used now in production environments.
– gpg-ezmlm is a set of scripts which add to the ezmlm package the ability to reencrypt messages sent to the list
– pgpdomo.tar.Z (213k), a set of replacement programs for Majordomo version 1.93 that will allow you to perform PGP encrypted administration and distribution of encrypted messages on selected lists while still retaining (mostly) normal operation of your cleartext lists
– SmailMan, an encrypted mailing list with Mailman, Exim and Gnupg on Debian GNU/Linux
– Shibboleth, a privacy and security aware mailing list manager, perfect for allowing closed groups to interact on the Internet without interference from outsiders and former insiders.
– Trunk, a program to manage e-mail discussion groups, or mailing lists. Trunk is completely managed via a web server (with PHP) and stores it’s data in an easy to manage SQL database