What is GNU Mailman?

``GNU Mailman'' (informally referred to as just ``Mailman''), is a system for managing electronic mailing lists. It is implemented primarily in Python, an object-oriented, very high-level, open source programming language. Mailing lists are administered by a list owner, and users can interact with the list - including subscribing and unsubscribing - through the web and through email. Site administrators can also interact with Mailman via a suite of command line scripts, or even via the interactive Python prompt. Mailman is the official mailing list manager of the GNU project and is available under terms of the GNU General Public License [GPL].

Mailman strives for standards compliance, and as such is interoperable with a wide range of web servers and browsers, and mail servers and clients. Of the web servers, it requires the ability to execute CGI scripts, and of mail servers it requires the ability to filter messages through programs. Apache is probably the most widely used web server for Mailman, and any of the Big 4 mail servers (Sendmail, Postfix, Qmail, and Exim) will work just fine. The HTML that Mailman outputs is extremely pedestrian so just about any web browser should work with it, as long as it supports cookies. Mailman should work with any MIME-compliant mail reader. Mailman works on any Unix-like operating system, such as GNU/Linux.

Mailman supports a wide range of features, such as:

• User selectable delivery modes. Members can elect to receive messages immediately, or in batches called digests. Two forms of digests are supported, RFC 1153 style plain text digests [RFC1153], and MIME multipart/digest style digests. Non-digest deliveries can be personalized specifically for the recipient of the message. This means that various aspects of the message, i.e. the header or footer, or the To field, can contain information specific to the member receiving the message.

• Extensive privacy options which allow a list administrator to select policies for subscribing and unsubscribing (open, confirmation required, or approval required), policies for posting to the list (open, moderated, members only, approved posters only), and some limited spam defenses.

• Automatic bounce processing. Bouncing addresses are the bane of any mailing list, and Mailman provides two mechanisms for automatic bounce detection, regular expression based bounce matching and Variable Envelope Return Paths [VERP].

  RFC 3464 [RFC3464] and the older RFC it replaces [RFC1894] describe a standard format for bounce notifications. However, many mail systems ignore or incorrectly implement this standard. For recognizing bounce messages, Mailman has an extensive set of regular expression based matches used to dig the bouncing address out of the notice. For fool-proof bounce detection, Mailman also supports VERP, a technique where the intended recipient's address as it appears on the mailing list is encoded into the envelope sender of the message. Because remote mail servers are required to send bounces to the envelope sender, Mailman can unambiguously decode the intended recipient's address and register an accurate bounce. Note that technically, VERP must be implemented in the mail server, but Mailman's use of the technique is close enough to warrant the label.

• Archiving. Mailman comes bundled with an archiver called Pipermail. Pipermail's chief advantages are that it comes bundled, that it is implemented in Python, and that in Mailman 2.1 it is internationalized, allowing the display of messages in alternative languages and character encodings. Its primary disadvantages are that it doesn't support searching and isn't very customizable. Mailman is easily integrated with external archivers.

• A mail to news gateway. Mailman can be configured to gateway lists to and from Usenet newsgroups. For example, the comp.lang.python newsgroup is gatewayed to the python-list@python.org mailing list. Even moderated lists, such as comp.lang.python.announce can be gatewayed, with Mailman serving as the moderation tool.

• Auto-responder, content filtering, and topics. The auto-responder can be set up to send a canned message whenever someone posts to the list, or emails the list owner or -request robot. Content filtering allows the list owner to explicitly filter or pass specific MIME (Multipurpose Internet Mail Extensions [RFC2045]) content types. Topics allow the list owner to assign incoming messages to any of a configurable number of groups, and members can ``subscribe'' to a specific topic, receiving only the subset of list traffic that matches the desired topics.

• Virtual domains. Mailman can be used on a mail server that supports multiple virtual domains. For example, the python.org and zope.org mail domains are run on the same machine, from the same Mailman installation. The one limitation in Mailman 2.1 is that a mailing list with the same name may not appear in more than one domain. This restriction will be lifted in future versions.

Mailman also provides each list with its own home page (called a ``listinfo'' page) which can be customized through the web. Mailing lists can be automatically created and deleted through the web (with proper support from the mail server). Mailman also provides web-based approval of moderated messages and subscriptions. There are a host of other smaller new features in Mailman 2.1 which won't be described in this paper.