I kinda.. super hate .. this feature in email and ticketing systems, so I’m not sure I want it in our product?
I mean maybe as a plugin perhaps. But this “automatically tell people I got their email” thing is something that I really do not want in Discourse proper.
Sorry if this is a bit too much radical honesty, but 
My main advice to you is disable PMs. That’s already a feature in your user profile. (And staff can obviously punch through that and send you PMs all they want, just not regular users.)