PowerMTA!

Zentyal is a native Microsoft Active Directory® implementation on Linux incorporating all the network services required in a small and medium business environment: Directory & Domain Server, Mail Server, Gateway and Infrastructure Server.

Zimbra Collaboration is an open, secure collaboration tool for email, calendaring, tasks and more. With the most innovative web application available today, Zimbra boosts the productivity of users on any desktop and dramatically reduces TCO compared to legacy platform vendors.

MailerQ is a high performance Mail Transfer Agent (MTA) and a very fast SMTP server, designed for delivering large volume email messages at very high speeds. MailerQ is fast. Its sole purpose is to send emails. Storing, queuing and prioritizing messages is done in a separate AMQP message broker RabbitMQ MailerQ gives full control over queued messages. Monitor email delivery and intervene when necessary. Prioriziting and routing messages has never been easier..

Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft. It is part of the Microsoft Servers line of server products and is widely used by enterprises using Microsoft infrastructure solutions. Exchanges major features consist of electronic mail, calendaring, contacts and tasks; support for mobile and web-based access to information; and support for data storage.

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 customized, enabled or disabled at will.

Atmail webmail system offers a webmail client, email server platform and mailserver appliance for Linux, Unix and Windows.

Sendmail is a general purpose internetwork email routing facility that supports many kinds of transfer and delivery methods, including the Simple Mail Transfer Protocol (SMTP) used for email transport over the Internet. A descendant of the delivermail program written by Eric Allman, Sendmail is a well-known project of the free and open source software and Unix communities, and has spread both as free software and proprietary software.

hMailServer is a free e-mail server for Microsoft Windows. Its used by Internet service providers, companies, governments, schools and enthusiasts in all parts of the world.

Powerful and simple SMTP/POP3/Finger server, with built in Web interface for accessing mail via Web browser. Supports security features, such as SMTP authentication. Has built-in TZO client for users without static IP address. Version 1.8.8.8 may include unspecified updates, enhancements, or bug fixes.

AtMail is an open source webmail client written in PHP. We aim to provide a elegant Ajax webmail client for existing IMAP mailservers, with less bloat and a focus on an intuitive, simple user interface.

Xeams is a secure & powerful mail server for Windows, Linux, Solaris, MacOSX and other flavors or UNIX, which supports SMTP, POP3, and IMAP. It includes a powerful spam/junk filtering engine that elimates up to 99% of junk email right out of the box. The software offers a flexible approach to email, making it a very user friendly server.