This is where the filter drops the incoming email before calling the
rest of the mailagent for processing it.  The filter, is small, fast,
and is called for every incoming mail; and is written in C. The rest
of mailagent is written in perl, and can be slow, and only one
instance is run for a whole batch of incoming mail. So, the filter
delivers mail here, where mailagent processes it, and delivers it into
the user spool.

arch-tag: b84bcad3-baef-4c57-b8bb-6fbfd50b46ff
