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 gMail 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