Explanation
The mailer.conf file is used to configure the mailer programs on a BSD system. It allows the system administrator to replace the default mailer programs, such as sendmail, with alternative ones, such as postfix or exim. The mailer.conf file contains a series of entries, each consisting of a label, an equal sign, and a command. The label is the name of the default mailer program, and the command is the path to the alternative mailer program that will be executed instead. For example, to replace sendmail with postfix, the mailer.conf file should contain the following entries:
sendmail /usr/local/sbin/sendmail send-mail /usr/local/sbin/sendmail mailq /usr/local/sbin/sendmail newaliases
/usr/local/sbin/sendmail hoststat /usr/local/sbin/sendmail purgestat /usr/local/sbin/sendmail The mailer.conf file is read by the mailwrapper program, which is a binary that acts as a wrapper for the mailer programs. The mailwrapper program is installed as /usr/sbin/sendmail and /usr/libexec/sendmail/sendmail, and it invokes the appropriate mailer program based on the mailer.conf file. Therefore, to replace the system MTA with another mail server program, the mailer.conf file needs to be updated with the path to the new MTA.
References:
Chapter 31. Electronic Mail | FreeBSD Documentation Portal
DragonFlyBSD: mta