The following is a list of additional documentation about email applications.
Information on configuring Sendmail is included with the sendmail and sendmail-cf packages.
/usr/share/sendmail-cf/README.cf — Contains information on m4, file locations for Sendmail, supported mailers, how to access enhanced features, and more.
In addition, the sendmail and aliases man pages contain helpful information covering various Sendmail options and the proper configuration of the Sendmail /etc/mail/aliases file.
/usr/share/doc/postfix-<version-number> — Replace <version-number> with the version number of Postfix. This directory contains a large amount of information about ways to configure Postfix.
/usr/share/doc/fetchmail-<version-number> — Contains a full list of Fetchmail features in the FEATURES file and an introductory FAQ document.
/usr/share/doc/procmail-<version-number> — Contains a README file that provides an overview of Procmail, a FEATURES file that explores every program feature, and an FAQ file with answers to many common configuration questions.
When learning how Procmail works and creating new recipes, the following Procmail man pages are invaluable:
procmail — Provides an overview of how Procmail works and the steps involved with filtering email.
procmailrc — Explains the rc file format used to construct recipes.
procmailex — Gives a number of useful, real-world examples of Procmail recipes.
procmailsc — Explains the weighted scoring technique used by Procmail to match a particular recipe to a message.
/usr/share/doc/spamassassin-<version-number>/ — This directory contains a large amount of information pertaining to SpamAssassin. Replace <version-number> with the version number of the spamassassin package.
http://www.redhat.com/mirrors/LDP/HOWTO/Mail-Administrator-HOWTO.html — Provides an overview of how email works, and examines possible email solutions and configurations on the client and server sides.
http://www.redhat.com/mirrors/LDP/HOWTO/Mail-User-HOWTO/ — Looks at email from the user's perspective, investigates various popular email client applications and gives an introduction to topics such as aliases, forwarding, auto-replying, mailing lists, mail filters, and spam.
http://www.redhat.com/mirrors/LDP/HOWTO/mini/Secure-POP+SSH.html — Demonstrates a way to retrieve POP email using SSH with port forwarding, so that the email passwords and messages are transferred securely.
http://www.sendmail.net/ — Contains news, interviews, and articles concerning Sendmail, including an expanded view of the many options available.
http://www.sendmail.org/ — Offers a thorough technical breakdown of Sendmail features and configuration examples.
http://www.postfix.org/ — The Postfix project home page contains a wealth of information about Postfix. The mailing list is a particularly good place to look for information.
http://catb.org/~esr/fetchmail/ — The home page for Fetchmail, featuring an online manual, and a thorough FAQ.
http://www.procmail.org/ — The home page for Procmail with links to assorted mailing lists dedicated to Procmail as well as various FAQ documents.
http://www.ling.helsinki.fi/users/reriksso/procmail/mini-faq.html — An excellent Procmail FAQ, offers troubleshooting tips, details about file locking, and the use of wildcard characters.
http://www.uwasa.fi/~ts/info/proctips.html — Contains dozens of tips that make using Procmail much easier. Includes instructions on how to test .procmailrc files and use Procmail scoring to decide if a particular action should be taken.
http://www.spamassassin.org/ — The official site of the SpamAssassin project.
Sendmail by Bryan Costales with Eric Allman et al; O'Reilly & Associates — A good Sendmail reference written with the assistance of the original creator of Delivermail and Sendmail.
Removing the Spam: Email Processing and Filtering by Geoff Mulligan; Addison-Wesley Publishing Company — A volume that looks at various methods used by email administrators using established tools, such as Sendmail and Procmail, to manage spam problems.
Internet Email Protocols: A Developer's Guide by Kevin Johnson; Addison-Wesley Publishing Company — Provides a very thorough review of major email protocols and the security they provide.
Managing IMAP by Dianna Mullet and Kevin Mullet; O'Reilly & Associates — Details the steps required to configure an IMAP server.
Red Hat Enterprise Linux Security Guide; Red Hat, Inc. — The Server Security chapter explains ways to secure Sendmail and other services.