There are a large number of additional source of sendmail information available on the Web.
For anyone intending to do any serious work with sendmail, the O'Reilly Sendmail Book is a
must buy. O'Reilly also do a smaller "Sendmail Desktop Reference" which is a
useful guide as an addition to the larger book.
As a final note, it is worth mentioning a few points. Sendmail is by no means the only MTA available. Those considering using sendmail may also want to look into qmail, Exim, Smail, Securemail (formerly Wietse Venema's vmailer), MMDF, or any of several others. Many people consider other mailers simpler to configure and work with than sendmail. However, it is estimated that sendmail handles around 75% of all Internet email. Any serious mail administrator will encounter sendmail, and often - it is without doubt the most widely used MTA in existence today.