Information on configuring Sendmail is included with the sendmail and sendmail-cf packages.

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> — Contains a large amount of information about ways to configure Postfix. Replace <version-number> with the version number of Postfix.

/usr/share/doc/fetchmail-<version-number> — Contains a full list of Fetchmail features in the FEATURES file and an introductory FAQ document. Replace <version-number> with the version number of Fetchmail.

/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. Replace <version-number> with the version number of Procmail.

When learning how Procmail works and creating new recipes, the following Procmail man pages are invaluable:

https://www.sendmail.org/ — Offers a thorough technical breakdown of Sendmail features, documentation and configuration examples.

https://www.sendmail.com/ — Contains news, interviews and articles concerning Sendmail, including an expanded view of the many options available.

https://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.

https://fetchmail.berlios.de/ — The home page for Fetchmail, featuring an online manual, and a thorough FAQ.

https://www.procmail.org/ — The home page for Procmail with links to assorted mailing lists dedicated to Procmail as well as various FAQ documents.

https://partmaps.org/era/procmail/mini-faq.html — An excellent Procmail FAQ, offers troubleshooting tips, details about file locking, and the use of wildcard characters.

https://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.

https://www.spamassassin.org/ — The official site of the SpamAssassin project.

Sendmail Milters: A Guide for Fighting Spam by Bryan Costales and Marcia Flynt; Addison-Wesley — A good Sendmail guide that can help you customise your mail filters.

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.