Mailman's source code is published in a publicly available revision control system called Bazaar available through the code hosting and open source development service called Launchpad. On June 22, 2007, we switched from using Subversion on SourceForge to the new repository, and while the old Subversion repository will still be available read-only, no new updates will be committed to it. Hosting the source code on Bazaar provides both the core developers and unofficial third party extensions much more freedom to hack on Mailman.
Here is more detail on how to develop Mailman code using Bazaar and Launchpad.
Here are a list of important official and unofficial branches.
Here are the collection of resources for people interested in the development of Mailman.
Google did a 2006 Summer of Codeprogram in 2006, and Mailman was a sponsor. See the Summer of Code page and Mailman 2.2 page for more information.
Mailman is a GNU project with the majority of the copyrights being held by the Free Software Foundation. We therefore request that developers who contribute code, assign their copyrights in their Mailman contribution to the FSF. To do this, you first need to submit a GNU copyright assignment request form containing some basic information, and then fill out the form that the FSF sends you. Please let us knowafter you've sent the second form so that we can track your contribution. The FSF often doesn't tell us in a timely manner when such forms have been received.
Here are some useful references:
RFCs and drafts: