Sendmail Consortium:The open-source version of the most popular SMTP server. Downloads and documentation.
A First Guide for E-mail Installation: A draft about the installation/configuration of sendmail.
Elandsys.com: Sendmail: Features a tutorial on how to install and configure it.
Explosion in a Punctuation Factory: Excellent tutorial on how sendmail's rewriting rules work. Written by Brian Costales, the author of the best sendmail book around, "Sendmail", published by O'Reilly and Associates.
Linux-Sec.net: Features minimum mail server hardening recommendations. Includes information on how to do it, antispam, filtering, open relay, antivirus, and statistics analyzer.
Sendmail FAQ: Frequently asked questions about sendmail.
Sendmail Forum at Tek-Tips: Sendmail technical support forums and mutual help system for computer professionals. Selling and recruiting forbidden.
Sendmail Inc.: Commercial Sendmail products and services.
Sendmail Routing with LDAP: Step-by-step instructions on using sendmail's LDAP routing feature. [Sun support contract required]
Sendmail V8: A (Smoother) Engine Powers Network E-mail: Excellent description of sendmail version 8 features.
Sendmail-8 Tips and Tricks by Andrzej Filip: Sendmail tips and tricks, including smarttable for selecting smart host based on envelope sender address.
Sendmail: Theory and Practice: Including how to order and the unofficial press kit.