O'Reilly: Sendmail. Second Edition, January 1997

Also, since the publication of the first edition, feedback from around the world made it clear that a number of improvements could be made. Some readers felt, for example, that the original offered too little in the way of real-world help, and that it lacked an annotated, major configuration file. Other readers offered suggestions of ways to make the book clearer. Some of these were minor but others, like the treatment of options, required extensive rewriting. One of the most noticeable changes between this edition and the original is the addition of section numbers and their attendant cross referencing.