Is the payment to get the email delivered or to get guaranteed response?

I'd prefer the latter for two reasons: 1) as a receiver I don't want to miss out on a potentially interesting email just because the sender couldn't pay (for technical or financial reasons), and 2) as a sender I'd like some sort of guarantee if I'm going to pay for something.

It's probably the more difficult approach of the two so I suggest testing both approaches and see what works best.

Congrats on getting this far already. Like you say it's not a new idea, but who cares about ideas. It's about execution anyway. Looking forward to the launch!