IndiMail is a messaging platform built using qmail and other messaging software. IndiMail has the ability to distribute the users in a domain across multiple hosts. IndiMail has multiple queues (called a queue collection), which can achiever very high concurrencies. The system IDs and configuration files used by IndiMail are all configurable through a set of environment variables, allowing you to customize IndiMail the way you want. IndiMail does not use hard-coded uid/gid in the executables, which makes it easy to distribute a binary package. IndiMail has features that any modern email server should have: SMTP, QMQP, QMTP, IMAP, POP3, DKIM, BATV, greylisting, filtering capability, and spam/virus control.
The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, Webmail, and mailing list services within a single consistent framework. Individual components can be enabled or disabled at will. Courier implements basic Web-based calendaring and scheduling services integrated in the Webmail module.
PigeonDeliver is the delivery agent of the PigeonAir project, a solution to easily build up clustered, scalable, and modular email services. PigeonDeliver is modular in nature, allowing it to be used on Postfix and other MTAs using their own native APIs without performance loss. It allows mail services to be easily and transparently distributed among a cluster of mail server, easily supporting new services and new modules added using the PigeonDeliver API, which simplifies writing filters and/or delivery agents.