Sendmail, 3rd Edition

Chapter 7

Section 7.1.4	The check_compat feature sets up the check_compat rule set to perform access database lookups (V8.12 and later).
Section 7.4	Prior to V8.9, relaying was allowed by default. Beginning with V8.9, relaying is turned off by default. In place of default relaying, V8.9 and later sendmail provide features, macros, and databases that set relay policy in a variety of ways.
Section 7.4.10	The sendmail program will refuse a mail message if the address specified as part of the SMTP MAIL FROM: has a domain part that cannot be looked up, unless the accept_unresolvable_domains feature is used (V8.9 and later).
Section 7.5	The access database was introduced in V8.9 sendmail, and improved upon in later versions. It provides a single central database with rules to accept, reject, and discard messages based on the sender name and address.
Section 7.5.3	The sendmail program now offers much finer control of addresses and rejections in the access database (V8.10 and later).
Section 7.5.4	You can change the default rejection message (to augment it or to clarify the reason for the rejection) using the confREJECT_MSG mc file macro (V8.12 and later).
Section 7.5.7	Using the check_compat feature, you can enable a rule set that makes decisions about envelope-sender and envelope-recipient pairs with entries in the access database (V8.12 and later).
Section 7.6	Beginning with V8.12, sendmail offers hooks to access external programs via sockets, and a library to ...