For the most part, we illustrate sendmail under BSD Unix and its variants (such as FreeBSD). Where AT&T System V (SysV) differs (such as Sun's Solaris 2.x), we illustrate those differences.
Our primary focus throughout this book is on V8.12 sendmail. For completeness, and where necessary, we also discuss V8.11 and earlier (such as BSD's version 5, IDA, early Sun, Ultrix, and NeXT).
 The versions jump from 5 to 8 because the managers of the BSD 4.4 Unix distribution wanted all software to be released as version 8. Prior to that decision, the new BSD sendmail was designated Version 6. V6 survived only the alpha and beta releases before being bumped to V8.