16.12 Dynamic packing

     Whenever a mail event is started, FD scans the messages in the
     netmail folder and possible packets are built. Messages that do
     not qualify to be processed will not be packed (but kept in the
     internal list). If a system calls in, FD will scan the message
     list stored in memory and create a mail packet on the fly and
     immediately send it to the calling system. So mail is always
     available, 24 hours/day.

     This means that your system does not have to be in an event for
     other systems to pickup their mail. Note that you can prevent
     systems from picking up their mail with the DENY command and by
     modifying an event's behavior.