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.
Engroup is an enterprise groupware system promotes collaboration among members in a team or company. It also helps members manage their business better. It has the following modules: a dashboard that aggregates contents from the mail system and RSS from some known sources; private messages, an alternative method for fast communication among members to replace chatting or email; email, which supports the IMAP protocol with SSL enabled; a forum, implemented with JForum; a wiki, implemented with JSPWiki; HRM to manage employee information; CRM to manage customer information; a document management system; a project management system; a todo list to track private tasks of a user; and an RSS reader.
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.
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.
smtp-cli is an SMTP command line client with support for advanced features such as STARTTLS, SMTP-AUTH, or IPv6. It also has scriptable message composition capabilities supporting anything from simple plain-text messages right up to building complex HTML emails with alternative plain-text part, attachments, and inline images. It's also a convenient tool for testing and debugging SMTP servers' setups. smtp-cli can be used to check a given server's capabilities, to test your server's setup, or to create and dispatch email messages. The command line interface is intuitive, and everything is scriptable and can run in a completely non-interactive mode from scripts or cron jobs. smtp-cli is ideal for shipping log files from remote machines, running periodic mail delivery test loops, etc.