Whaaaaaaat
So you can whitelist/blacklist, or is it a blanket thing? That wouldn't work for me.
Can't find info on the email service though
nostr:npub1n0delesspp5qyp9sd6d4fql9g3s96k7g4yyvfmpdncpp542wvxzqksp427 has this feature. I tested by sending an email to thebullishbitcoiner@nodeless.io and received an email response with the following invoice. The minimum paywall is 1000 sats.
You can check it out here:
https://nodeless.io/?ref=b42efe7e-7efb-47ec-b2dc-3421f24f331f 
Discussion
It should be under “Nodeless Address” in your dashboard. It’s a blanket thing it seems. If the sender pays the fee, the email is forwarded to the final destination email. If not, it stays in purgatory. 😄 