Two topics that are only touched upon in this book are the Domain Name System (DNS) and TCP/IP network communications. At a typical site, a significant number of mail-related problems turn out to be problems with one of these other areas rather than with sendmail.
The DNS is well documented in the book DNS and BIND, 4th Edition by Paul Albitz and Cricket Liu (O’Reilly & Associates, Inc., 2001).
The protocols used to communicate over the Internet are well documented in the book TCP/IP Network Administration by Craig Hunt (O’Reilly & Associates, Inc., 1992).
Finally, many mail problems can be solved only by the system administrator. The sendmail program runs as root and can be installed and managed only by root. The art of functioning effectively as root is superbly covered in the UNIX System Administration Handbook by Evi Nemeth, Garth Snyder, Scott Seebass, and Trent R. Hein (Prentice Hall, 3rd Edition, 2000).