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.
rjmailer is a replacement for the SMTP part of Sun's javamail library. It sends email messages via SMTP and has the ability to connect directly to the receiving mail servers instead of using a relay server, which in many cases can result in more detailed information about the delivery status.
Why a milter archiver? If you are required to also log source and destination IPs and BCCs (additional rcpt to's not in a CC field communicated at the SMTP protocol level), then the only option is do this in the MTA, or in a milter application that has hooks in the MTA process (which is exactly why milter was invented). It uses a simple regex file that can be used with several *source target-mailbox rules to define what gets mirrored where. The archiver tries to be as quiet as possible. The possible methods to archive are SMTP, sendmail inject / Postfix inject, IMAP delivery, and milter-add recipient. It was build as a replacement for a Postfix BCC setup: (sender_bcc_maps = regexp:/etc/postfix/archive; recipient_bcc_maps = regexp:/etc/postfix/archive).
Lamson is an SMTP server adopts a modern Web application framework design. Rather than bizarre flat file "databases" and hashtable stores, Lamson uses anything that Python does to store data, including Django's ORM, web.py's simple database library, Tokyo Tyrant, Lightcloud, Python's Shelve (not recommended), Raw SQLite3, SQLObject, CouchDB, Mongo DB, and SQLAlchemy.
HeaderCleaner is a small SMTP server plugin that will only allow a set of predefined headers in an outgoing email message and will strip all other headers from the message. This is meant to improve your privacy and better protect the mail handling server. This version is made for Communigate-Pro, but can be adapted for other systems.
CleanMail is a complete SMTP/POP3 email security solution that fights against spam and viruses. It works with any mail server like Microsoft Exchange 2003/2007/2010, Microsoft Small Business Server (SBS) 2003/2008, Lotus Notes/Domino, or IMail. CleanMail includes a comprehensive set of filters, including an attachment blocker, remote blacklist filter (DNSBL), and a complete build of SpamAssassin.
OpenSMTPD is an RFC compliant, secure, and simple SMTP server ported from OpenBSD to Linux. It supports the SMTP protocol, including SSL versions for added security, virtual domains, forward files, and external delivery agents like procmail, and can do most of the basic things that other MTA implementations do.
Rumble is a mail server suite for SMTP (ESMTPSA), HTTP, POP3, and IMAP4v1 with an extensive C/C++ and Lua API for scripting. It comes with support for SSL/TLS, SQLite, and MySQL and has a Web-based administration and Web mail feature. Additional included modules feature greylisting, DNS blacklisting, SpamAssassin support, and dynamic performance scaling as well as a multithreaded structure that allows for clustering of multiple servers for one or all domains. With the Lua API you can create your own new server mailing lists, CRM software, e-marketing, and other extensions within minutes.