The information about the registered handlers is stored in the SQLite
database handlers.db, which is located
in /var/lib/plesk/mail/handlers/. The registration of the handler
means that MTA will use it to filter mail.
For the Qmail MTA, the code which invokes handlers is either patched
into qmail or installed as wrappers around qmail-queue, qmail-local, and
qmail-remote.
For the Postfix MTA, the code which invokes handlers is executed via the
specific Milter service psa-pc-remote.
with