Mail

Sendmail:

• Sendmail.org
• other sendmail hints and other email sources, and Sendmail Installation and Operation Guide (from the sendmail distribution).
• Also, a general introduction at `Explosion in a Punctuation Factory'
• Securing Sendmail8 against relay

Exim:

• Home
• Documentation

MUAs:

• Pine (PGP, Q&A, other pine help)
• Eudora (the newest PGP distribution has support for Eudora, but there are other security plugins available)
• Pegasus (and PGP interfaces for it)
• There's a program called PGPClick, which seems to provide an interface to PGP for general Windows programs (ie, it doesn't work via a plugin). It might be worth investigating.
• MH & nmh: Email for Users & Programmers (frameless toc), and MH FAQ, mh-e toc
• Also IMAP, includes a products database

Misc:

• Majordomo distribution and FAQ
• MHonArc: mail to HTML.