This is excellent stuff for people like me, seeking a basic understanding of how sendmail works and a guide to simple administration tasks. There may not be much in it for serious gurus, but they will appreciate the second half of Sendmail, which is a detailed reference manual, containing far more than most people will ever want to know (I have only glanced at it myself). This could, perhaps, have been printed as a separate book: not only are some people likely to want just the reference manual or just the tutorial and administration guide, but bundling them together makes for an awkwardly thick volume. (O'Reilly also publish a pocket-sized Sendmail Desktop Reference, which is basically a summary of and index into the reference manual in Sendmail.)
This second edition of Sendmail covers version 8.8, but most of it (and certainly the tutorial sections) won't date that rapidly. If you actually administer sendmail then it is an obvious O'Reilly title to add to your collection, but it may also interest curious users — after all, almost everyone uses sendmail, even if indirectly.
Note: there is now a 3rd (2002) edition of Sendmail.