THIS IS OUTDATED - read the docs which come with sendmail
INSTALLATION AND OPERATION GUIDE
For Sendmail Version 8.9
Sendmail\uTM\d implements a general purpose internetwork mail routing facility under the UNIX© operating system. It is not tied to any one transport protocol -- its function may be likened to a crossbar switch, relaying messages from one domain into another. In the process, it can do a limited amount of message header editing to put the message into a format that is appropriate for the receiving domain. All of this is done under the control of a configuration file.
Due to the requirements of flexibility for sendmail, the configuration file can seem somewhat unapproachable. However, there are only a few basic configurations for most sites, for which standard configuration files have been supplied. Most other configurations can be built by adjusting an existing configuration file incrementally.
Sendmail is based on RFC821 (Simple Mail Transport Protocol), RFC822 (Internet Mail Headers Format), RFC1123 (Internet Host Requirements), RFC2045 (MIME), RFC1869 (SMTP Service Extensions), RFC1652 (SMTP 8BITMIME Extension), RFC1870 (SMTP SIZE Extension), RFC1891 (SMTP Delivery Status Notifications), RFC1892 (Multipart/Report), RFC1893 (Mail System Status Codes), RFC1894 (Delivery Status Notifications), RFC1985 (SMTP Service Extension for Remote Message Queue Starting), and RFC2033 (Local Message Transmission Protocol). However, since sendmail is designed to work in a wider world, in many cases it can be configured to exceed these protocols. These cases are described herein.
Although sendmail is intended to run without the need for monitoring, it has a number of features that may be used to monitor or adjust the operation under unusual circumstances. These features are described.
Section one describes how to do a basic sendmail installation. Section two explains the day-to-day information you should know to maintain your mail system. If you have a relatively normal site, these two sections should contain sufficient information for you to install sendmail and keep it happy. Section three describes some parameters that may be safely tweaked. Section four has information regarding the command line arguments. Section five contains the nitty-gritty information about the configuration file. This section is for masochists and people who must write their own configuration file. Section six describes configuration that can be done at compile time. The appendixes give a brief but detailed explanation of a number of features not described in the rest of the paper.
DISCLAIMER: This documentation is under modification.