The Mail Transfer Agent (MTA) included in the FreeBSD base system is sendmail, which is an aincent (first shipped with 4.1BSD in 1983) MTA that can be very tedious to configure. To quote Wikipedias sendmail article:
Sendmail can be difficult to configure. Whereas nearly all popular Unix based server software has human readable configuration files, Sendmail's configuration is not considered human readable. Instead, the Sendmail authors recommend that administrators learn and use macro language tools, particularly M4 to configure Sendmail. Sendmail is unique amongst Unix based MTAs in this requirement, and no MTA developed since Sendmail requires or recommends the use of macro languages for configuration.
This category is about me trying to make my life easier by documenting how to get sendmail to behave like I want it to, on the servers where it is impossible for me to replace it with Postfix.