As we mentioned above the best reference of all is the “bat” book, a.k.a. sendmail by Bryan Costales and Eric Allman. The sendmail Desktop Reference is also available (note: the Desktop Reference is small and almost fits in a shirt pocket).
There are hundreds of online references for sendmail. We will only include a few of them here as the rest of them are available to you via a simple Web search.
New check_* rulesets/patches for sendmail 8.9
Making Archival Copies Of All SMTP Messages On An SMTP Relay
A Highly Scalable Electronic Mail Service Using Open Systems
Sendmail address rewriting mini-HOWTO
The Linux Electronic Mail Administrator HOWTO
Linux off-line mailing method (offline mailaddr with 1 account)