Name: notify_external
Type: STRING
Default-Value: NULL
Last-Modified: 2.4.0

Path to the external program that notifyd(8) will call to send mail
notifications.

The external program will be called with the following command line
options:

    .. option:: -c    class

    .. option:: -p    priority

    .. option:: -u    user

    .. option:: -m    mailbox

And the notification message will be available on *stdin*.
