Protonmail + a VPN paid for with a cash purchased prezzy card.

Next level paranoia would be either a disposable machine/phone to send from, or run from a linux boot CD with no persistent storage.

That will get you essentially perfect anonymity. You're more likely to be identified by phrasing and vocab analysis.

You could always translate into another language and then back into English through different services to make that harder.

Cheers - N