@HardenedSteel this is a niche still awaiting for its entrepreneur.
Proton accepts btc (sigh (put +1 to the column that argues for them being a honeypot)). You might use trocador to exchange from btc to xmr and make your payment.
But, again, some sort of email service that takes XMR in exchange of service would be good. The operators of the email service can even use the XMR payment as a sort of counter-spam measure against bot accounts, and spam senders. The service can also use "Mullvad-style" random digits per the customer in order to track their XMR payments for the service, and demand no personally identifiable information, at all.
>cannot use their tor hidden service for anonymous account creation
>cannot use the btc payment option during anonymous account creation
>no XMR payment option at all
I think Monero community can do better. Just create a version of cockli service that forces people to pay a buck a month in XMR. Promise to keep their emails encrypted in the server SSDs, or allow them to use POP to pull their emails to their local devices. etc. etc.