Integration with mail service is performed via mail handlers.
Handler is an executable file which is invoked at selected points (called hooking points).
Mail handlers are used to handle incoming, outgoing, and pass-through mail at various stages of processing by the Mail Transfer Agent (MTA). One or more handlers can be applied to a message.
Leave your feedback on this topic here
If you have questions or need support, please visit the Plesk forum or contact your hosting provider.
The comments below are for feedback on the documentation only. No timely answers or help will be provided.