The directory /etc/mail contains all the human-friendly Sendmail configuration files, plus a couple human-hostile ones. For most common tasks on a server that accepts incoming email, you only need to worry about four files: /etc/mail/access, /etc/mail/aliases, /etc/mail/mailertable, and /etc/mail/relay-domains.
The access file lets you set per-host and per-domain access controls for your mail server.
The aliases file contains a map of email redirections for the local host.
The mailertable allows you to override MX records for this mail server. The mailertable is notably useful when you're using your FreeBSD system to provide an additional layer of security for a proprietary system with a poor security record, such as Microsoft ...