I can't recall which service, it's right on the tip of my tongue, but I've seen it done before that such a reply would end up being routed to the correct user.

I can't recall which service, it's right on the tip of my tongue, but I've seen it done before that such a reply would end up being routed to the correct user. In the simplest case you get "X Commented on your Post!" sent from your service's account but with reply-to set to X's address. If you wanted to obscure X's address, you could process replies to your service's address and route them accordingly.