Mailagent can deliver mail to plain UNIX folders (also known as "mbox format"),
to MMDF folders, to MH folders (with unseen sequence update built-in and using
locks, not like rcvstore which does not!) or to directories (ala MH, but
without unseen sequence support and with alternate naming possible).

It also supports delivery to folders with the "x" bit set, in which case
mailagent interprets those as being hooks. It either pipes the message
to the "program" or further interprets the hook to do more processing.
See the manual page mailagent(1) under the section "MAIL HOOKS" for more
details.