The information about the registered handlers is stored in the SQLite
handlers.db, which is located
/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