Messaging

Sendmail

Sendmail provides control of the mail stream and of the messaging system, protecting against spam, viruses, and intrusion while giving legitimate users secure message access.

qmail

qmail is an Internet Mail Transfer Agent (MTA) for UNIX-like operating systems. It's a drop-in replacement for the Sendmail system provided with UNIX operating systems. qmail uses the Simple Mail Transfer Protocol (SMTP) to exchange messages with MTA's on other systems.

Postfix

Postfix attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users. Thus, the outside has a sendmail-ish flavor, but the inside is completely different.

Mailman

Mailman is free software for managing electronic mail discussion and e-newsletter lists. Mailman is integrated with the web, making it easy for users to manage their accounts and for list owners to administer their lists. Mailman supports built-in archiving, automatic bounce processing, content filtering, digest delivery, spam filters, and more.

MailScanner

A Free Anti-Virus and Anti-Spam Filter. Protecting over 5 billion e-mails every week, for many tens of millions of users.

Jabber

Jabber is a set of streaming XML protocols and technologies that enable any two entities on the Internet to exchange messages, presence, and other structured information in close to real time.

OPEN-XCHANGE

The OPEN-XCHANGE Collaboration and Integration Server Environment allows you to store appointments, contacts, tasks, mails, bookmarks, documents, and many more elements and share them with other users. This environment can be accessed via any modern web browser and multiple fat clients like KDE Kontact, Apples iCAL, Konqueror, Mozilla Calendar, any many more, based on open standards and interfaces. If you download OXlook, then OPEN-XCHANGE can also be accessed via MS Outlook and Palm devices.