The GNU Mailing List Manager, which manages email discussion lists. Mailman gives each mailing list a web page, and allows users to subscribe, unsubscribe, etc. over the web. The list manager can administer his or her list entirely from the web.

Mailman also integrates most things people want to do with mailing lists, including archiving, mail <-> news gateways, and so on. It has all of the features you expect from such a product, plus integrated support for the web (including web based archiving), automated bounce handling and integrated spam prevention.

Note that this package contains the legacy (2.x) branch of Mailman. All new development happens in the Mailman 3 suite, available in Debian via the mailman3 metapackage.