GNU Mailman is a computer software application from the GNU project for managing electronic mailing lists.
Mailman is coded primarily in Python and currently maintained by Barry Warsaw. Mailman is free software, distributed under the GNU General Public License.
While I currently don't have a Mailman installation installed in the BedroomProgrammers.net, I have had to configure it on several occasions so keeping my config guide handy in my documentation system could help me in the future.
Mailman in it's lowest form is a mail relay. Improperly configured, this means that it could be used as a spam relay. This is not something that can be solved through crafty firewall trickery so care needs to be taken when configuring the service.
Mailman needs to be able to both send and receive mail. These jobs are handled by the system's MTA, the only two of which I use are Postfix and Sendmail depending on the system. If I ever get around to it I'll also get around to writing a config guide for Exim. Refer to those for proper hardening and firewalling.
The other half of Mailman is the administrative interfaces. This is done through a web site hosted on an Apache installation. Please refer to that guide for firewall and configuration on that service.