Many users initially find Sendmail difficult to configure, primarily due to the large number of options available. Access to additional Sendmail documentation can be very helpful, especially when setting configuration options.
The best sources of information about how to configure Sendmail are included with the sendmail and sendmail-cf packagess.
/usr/share/doc/sendmail/README.cf — Contains information on m4, file locations for Sendmail, supported mailers, how to access enhanced features, and much more.
/usr/share/doc/sendmail/README — Contains information on the Sendmail directory structure, IDENT protocol support, details on directory permissions and the common problems these permissions can cause if misconfigured.
http://www.sendmail.net — News, interviews and articles concerning Sendmail, offering a larger view of the many options available.
http://www.sendmail.org — Offers a thorough technical breakdown of Sendmail features and configuration examples.
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.