Sendmail is one of the most widely used MTAs available. Although Sendmail can be somewhat difficult to set up and maintain, it has several m4 macros already written that can perform many functions in the fight against spam. Information on Sendmail can be found on the web at http://www.sendmail.org/.
Updating the Configuration
Before we get into specific methods for controlling spam with Sendmail, some background coverage for manipulating the configuration files under Sendmail is required. Please note that these steps will need to be performed in order to enable each of the methods outlined for Sendmail. These instructions should be good for any recent version of Sendmail (anything above version 8.10).
The Sendmail configuration file ...