ezmlm lets users set up their own mailing lists within qmail's address
hierarchy. A user, Joe, types

ezmlm-make ~/SOS ~/.qmail-sos joe-sos isp.net

and instantly has a functioning mailing list, joe-sos@isp.net, with all
relevant information stored in a new ~/SOS directory.

ezmlm sets up joe-sos-subscribe and joe-sos-unsubscribe for automatic
processing of subscription and unsubscription requests. Any message to
joe-sos-subscribe will work; Joe doesn't have to explain any tricky
command formats. ezmlm will send back instructions if a subscriber sends
a message to joe-sos-request or joe-sos-help.

ezmlm automatically archives new messages. Messages are labelled with
sequence numbers; a subscriber can fetch message 123 by sending mail to
joe-sos-get.123. The archive format supports fast message retrieval even
when there are thousands of messages.

ezmlm takes advantage of qmail's VERPs to reliably determine the
recipient address and message number for every incoming bounce message.
It waits ten days and then sends the subscriber a list of message
numbers that bounced. If that warning bounces, ezmlm sends a probe; if
the probe bounces, ezmlm automatically removes the subscriber from the
mailing list.

ezmlm is easy for users to control. Joe can edit ~/SOS/text/* to change
any of the administrative messages sent to subscribers. He can remove
~/SOS/public and ~/SOS/archived to disable automatic subscription and
archiving. He can put his own address into ~/SOS/editor to set up a
moderated mailing list. He can edit ~/SOS/{headeradd,headerremove} to
control outgoing headers. ezmlm has several utilities to manually
inspect and manage mailing lists.

ezmlm uses Delivered-To to stop forwarding loops, Mailing-List to
protect other mailing lists against false subscription requests, and
real cryptographic cookies to protect normal users against false
subscription requests. ezmlm can also be used for a sublist,
redistributing messages from another list.

ezmlm is reliable, even in the face of system crashes. It writes each
new subscription and each new message safely to disk before it reports
success to qmail.

ezmlm doesn't mind huge mailing lists. Lists don't even have to fit into
memory. ezmlm hashes the subscription list into a set of independent
files so that it can handle subscription requests quickly. ezmlm uses
qmail for blazingly fast parallel SMTP deliveries.

ezmlm-idx originated as an add-on to ezmlm. It now exists as a complete package on its own, but can still be considered essentially as an extension to ezmlm. It adds multi-message threaded message retrieval from the archive, digests, message and subscription moderation, and a number of remote administration function. It modifies the configuration program ezmlm-make(1) so that it uses a text file template rather than compiled-in texts in list creation. In this manner, ezmlm-idx allows easy setup of lists in different languages and customization of default list setup. ezmlm-idx also adds MIME handling, and other support to streamline use with languages other than English. Prior to version 7, ezmlm-idx existed as an ezmlm source add-on, and as such did not work without ezmlm. ezmlm-idx tries to be compatible with ezmlm as much as possible in its usage, though the internal structure has changed considerably. ezmlm-idx also modifies the ezmlm subscriber database to be case insensitive to avoid many unsubscribe problems.
The IDX patches add: Indexing, (Remote) Moderation, digest, make

Fred Lindberg
Fred B. Ringel
Bruce Guenter
And many others

Project Homepage:
https://untroubled.org/ezmlm/