7.1.3 The @ event

         The @ event is active whenever your system is not in another
         mail event. The @ event is required. You do not have to tell
         the Mailer to send mail during the @ event, but it must be
         defined.

         You can use it to send local and/or low cost mail whenever
         the Mailer is not busy, etc. You can specify a schedule block
         (see Mail routing), using the @ tag and define routing rules
         for it just like any other mail event. If the Mailer does not
         find a schedule block for the @ tag, it will still use any
         global rules defined in the route file.