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Linux eMail Server - Mail Transfer Agent (MTA) software

Internet mail is broken down into three basic parts:

  1. MTA or Mail Transfer Agents - sendmail, qmail, etc - Links/Info
  2. Mail Retrieval Software - pop3, imap, etc - Links/Info
  3. Mail User Agent (MUA): E-Mail client - Netscape, MUTT, etc - Links/Info

Internet mail is sent from the email client to the email server which routes it to the intended destination which will also be an email server. The protocol by which the email servers communicate is called SMTP (Simple Mail Transfer Protocol) and the email server software is known as the MTA or Mail Transfer Agent. This page lists the popular MTA eMail server programs used, as well as supporting programs such as email list servers and spam filters. Also listed are the RFC's (Request For Comments) on which the internet email standards are currently or eventually based.

Linux email server configuration tutorials:
Mail Transfer Agents (MTA):

Open Source:

Commercial MTA's:

  • Axigen: eMail server, anti-virus, anti-spam, web mail, backup/restore.
  • Novell/S.U.S.E. Linux Openexchange Server (SLOX) - Postfix, IMAP and LDAP configured for use with MS/Outlook (See the YoLinux LDAP tutorial and see how to do it for free.) Supports MAPI (Messaging Application Programming Interface) clients and SIP. Third party support for anti-virus and anti-spam. Also POP3, IMAP4 support.
  • IBM: Lotus Notes Domino Mail Server - E-mail, Web access, Calendaring and Scheduling, bulletin boards, newsgroups, mobile support
  • Gordano - Messaging, Calendaring, Webmail, MAPI, anti-virus and anti-SPAM. Also POP3, IMAP4 support.
  • Sendmail.com - commercial version of sendmail
  • Scalix - Messaging, Webmail, MAPI. Third party support for anti-virus and anti-spam. Also POP3, IMAP4 support.
  • Stalker software: CommuniGate Pro - Features anti-Spam, clustering, web admin, web mail, multi-domain, Mail lists, LDAP, ACAP, SSL, CLL, SASL, anti-virus, IM, MAPI, POP3, IMAP4
  • Axigen mail server - SMTP, POP3, IMAP4, WebMail, List Server SSL/TLS, SPF, antivirus and antispam support

MTA Server support software:

  • OpenGroupWare.org - Was SKYRIX groupware server, then OGo. Piggybacks on an existing SMTP server to handle mail but then provides calendar groupware and mail interface with email clients. Uses WEBDAV interface protocol.


Open Relay Check:

Execute the following from your SMTP server:

   telnet relay-test.mail-abuse.org
This site will run a series of relay tests and return the results in the telnet session. The proper diagnosis for each test of course is "Relay access denied" and the final reply: "System appeared to reject relay attempts".

The following web sites can also help you perform an open relay test:

Mailing Lists:

SMTP dialog:

This is a sample of the dialog an e-mail client makes when connecting to an SMTP server for sending mail:

   telnet server-name 25     - SMTP communicates on port 25. See: /etc/services
   HELO your-domain          - This identifies the source of the mail.
   HELP                      - List the SMTP commands that are supported. (Included FYI and not part of a typical dialog)
   MAIL FROM: <your-email-address>
   RCPT TO: <recipient-email-address>
   DATA                      - End of DATA section is punctuated with a single dot on it's own line.
   Subject: E-mail-Subject
   Text of e-mail goes here
   .                         - The single dot

SMTP return codes let the e-mail client know if all went well.

See: RFC 822 for more information.

Mail headers and tracking a spammer:

Return-Path: <sender-of-email@domain-of-sender.com>    - This is the information that they provide and may easily be forged
Received: from domain-of-spammer-mail-server.com (MTA-node-name.domain-of-spammer-mail-server.com [XXX.XXX.XXX.XXX])     - Where XXX.XXX.XXX.XXX is the IP address
        by MTA-node.domain-name-of-server-receiving-spam.com (8.11.6/8.11.6) with ESMTP id fAR2XXXX2155                    of the MTA relay server used by the spammer. Block this IP.
        for <victim@isp.com>; Mon, 26 Nov 2001 20:04:23 -0600
Received: from domain-of-sender.com ([XXX.XXX.XXX.XXX]) by MTA-node-name.domain-of-spammer-mail-server.com - Where XXX.XXX.XXX.XXX is the IP address of the computer which sent
 (iPlanet Messaging Server 5.1 (built May  7 2001))                                                          the email to the MTA relay. Block this as well if you want to double protect yourself.
 with ESMTP id <0GNG0XXXXXXXVQ@MTA-node-name.domain-of-spammer-mail-server.com> for
 victim@isp.com; Tue, 27 Nov 2001 06:10:28 -0600 (CST)
Date: Tue, 27 Nov 2001 06:04:36 -0600
From: Claimed Name of Spammer <sender-of-email@domain-of-sender.com>
Subject: Re: FYI
To: victim@isp.com
The information within the "[ ]" is fairly reliable. Everything else can be set in the MTA configuration.


  • RFC 2645 - ON-DEMAND MAIL RELAY (ODMR) SMTP with Dynamic IP Addresses
  • RFC 2554 - SMTP Service Extension for Authentication
  • RFC2505 - Anti Spam
  • RFC 2487 - SMTP Service Extension for Secure SMTP over TLS
  • RFC 2442 - Batch SMTP Media Type
  • RFC 2197 - SMTP Service Extension for Command Pipelining
  • RFC 2034 - SMTP Service Extension for Returning Enhanced Error Codes
  • RFC 1985 - SMTP Service Extension for Remote Message Queue Starting
  • RFC 1891 - SMTP Service Extension for Delivery Status Notifications
  • RFC 1870 - SMTP Service Extension for Message Size Declaration
  • RFC 1869 - SMTP Service Extensions
  • RFC 1846 - SMTP 521 Reply Code
  • RFC 1845 - SMTP Service Extension for Checkpoint/Restart
  • RFC 1830 - SMTP Service Extensions for Transmission of Large and Binary MIME Messages
  • RFC 1652 - SMTP Service Extension for 8bit-MIMEtransport
  • RFC 1428 - Transition of Internet Mail from Just-Send-8 to 8bit-SMTP/MIME

Books on internet email programming: (Amazon)



by Bryan Costales, Eric Allman
ISBN #1565928393, O'Reilly & Associates 3 edition (December, 2002)

The most comprehensive book on Sendmail.

"Sendmail Cookbook"
by Craig Hunt
ISBN #0596004710, O'Reilly & Associates 1 edition (December, 2003)

"Sendmail Desktop Reference"
by Bryan Costales, Eric Allman, Gigi Estabrook (Editor)
ISBN #1565922786, O'Reilly & Associates

"Sendmail: Theory and Practice"
by Frederick M. Avolio, Paul Vixie (Contributor)
ISBN #1555581277, Butterworth-Heinemann

"Linux Sendmail Administration"
by Craig Hunt
ISBN #0782127371, Sybex

by Richard Blum
ISBN #0672321149, Sams

"Linux Email"
by Black, Koetter, Hilderbrandt, McDonald, Rusenko and Taylor.
ISBN #190481137X, Packt

I reviewed this book and found it to be a very complete discussion with examples of the installation, configuration, integration and use of Postfix, Courier POP/IMAP, SquirelMail, SpamAssassin, ClamAV and Cyrus SASL. It is concise and to the point. How to configure a Linux email server for the most popular and useful server configurations. This book is specific to the components listed and will get you up and running fast.

by Philip Hazel
ISBN #0596000987, O'Reilly & Associates

"An Introduction to Qmail"
by Dave Sill
ISBN #1893115402, APress

"Running qmail"
by Rich Blum, Richard Blum
ISBN #0672319454, Sams

"Stopping Spam"
by Alan Schwartz, Debby Russell (Editor), Simson Garfinkel
ISBN #156592388X, O'Reilly and Associates

"Removing the Spam : Email Processing and Filtering"
by Geoff Mulligan
ISBN #0201379570, Addison-Wesley Networking Basics Series

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