openICEM is a highly scalable server software that aims to provide a certified email service as specified by Italian law. It is based around the Postfix MTA, python-ldap, and python-clamd. It makes use of XML-RPC for communication among servers and does not require any shared service or shared filesystem.
DavMail is a POP/IMAP/SMTP/Caldav/Carddav/LDAP exchange gateway allowing users to use any email/calendar client (e.g. Thunderbird with Lightning or Apple iCal) with an Exchange server, even from the Internet or behind a firewall through Outlook Web Access. DavMail now includes an LDAP gateway to Exchange global address book and user personal contacts to allow recipient address completion in the mail compose window, and full calendar support with attendees free/busy display.
S22.Imap is an easy-to-use and well-documented .NET library component for communicating with and receiving electronic mail from an Internet Message Access Protocol (IMAP) servers. It supports IMAP IDLE notifications and IMAP over SSL, allows selective fetching of parts of mail messages, is inherently thread-safe, and is well documented with lots of example code.
Sendooway is a multi-user and multi-target ESMTP proxy that acts as mail transfer agent. Instead of configuring SMTP servers and authentication data for every mail identity on every device, it allows you to configure them centrally. Sendooway automatically uses the correct smarthost, logs on with the user's account data, and sends their mail.
Exim4U is a Web-based user interface and integrated software configuration for the Exim message transfer agent (MTA). Using any modern Web browser, you can use it to manage Exim email domains, user accounts, aliases, forwarders, and much more. Exim4U is a derivative work of the Virtual Exim (Vexim) project.
DKIM-Verify is a software application that can provide DKIM verification for non-DKIM-aware MDAs. Using DKIM-Verify, which functions as a "wrapper" for the MDA, mail originating from and destined for the local domain can be verified. In addition, DKIM-Verify can integrate with mailing list software to provide verification of messages received for later distribution.
Masqmail is an MTA (mail transport agent) for notebooks, workstations, home servers, and generally any host with no permanent network connection. It has special support for multiple transfer routes (e.g. to different mail providers). Address rewriting can be done in several ways. It is a slim replacement for full-blown MTAs such as sendmail, exim, qmail, or postfix and for the intended uses, configuration is much simpler.