Simple Mail Transfer Protocol is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP was first defined by RFC 821 (1982, eventually declared STD 10), and last updated by RFC 5321 (2008) which includes the Extended SMTP (ESMTP) additions, and is the protocol in widespread use today.
SMTP uses TCP port 25. The protocol for new submissions (MSA) is effectively the same as SMTP, but it uses port 587 instead. SMTP connections secured by SSL are known by the shorthand SMTPS, though SMTPS is not a protocol in its own right.
Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. It is freely available under the terms of the GNU General Public Licence. There is a great deal of flexibility in the way mail can be routed, and there are extensive facilities for checking incoming mail. Exim can be installed in place of Sendmail, although the configuration of Exim is quite different.
All versions of Exim previous to version 4.x are now obsolete and everyone is very strongly recommended to upgrade to a current release. The last 3.x release was 3.36. It is obsolete and should not be used.
Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers electronic mail, intended as an alternative to the widely used Sendmail MTA.
Originally written in 1997 by Wietse Venema at the IBM Thomas J. Watson Research Center and first released in December 1998, Postfix continues as of 2013 to be actively developed by its creator and other contributors. In January 2012 in a study performed by E-Soft, Inc., approximately 23% of the publicly reachable mail-servers on the Internet ran Postfix.
Is a general purpose internetwork email routing facility that supports many kinds of mail-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. It has spread both as free software and proprietary software.
Microsoft Exchange server was developed in 1993 when XENIX system was changed into exchange server. The next edition came out in 1997 and was called the Exchange Server 5.0. Since then editions such as Exchange server 2000, 2003, 2007 and 2010 have come out each better than its predecessor.
Exchange offers email, contacts, shared calendars and other functionality. On the Windows side Outlook is the application for Exchange. On the Mac side Entourage/Outlook and Mail can access Exchange servers. AppleMail only supports Exchange mail. Exchange comes in different flavors – in Microsoft style in year versions. There are currently versions 2007, 2010 and 2013. Under the hood Exchange can use POP, IMAP, and SMTP. In 10.5 Leopard Mail could talk to Exchange servers only via IMAP and SMTP. Snow Leopard added native support to talk to an Exchange server directly. Lion added support for Exchange 2010. Version 2013 is quite new.