Okay, because I had an idea... I was trying to think of a way that didn't require both the receiver AND the sender to have the client that had support for requiring a dm fee. Only the receiver needs to have the client that support paid dms if you do it like this I think.

-sender using any client sends mail to me

-since his npub isn't on my whitelist, my client autoreplies via DM with a lightning invoice set to whatever fee I set it at.

-he pays invoice

-my client let's me view message

Since on his end, it's all happening through DMs, it should work no matter what he uses.