Not technical/ needs to be super easy for everyone
If you're technical, use an anonymous remailer.
https://www.gnu.org/software/mailman/index.html
If you're less technical, use https://github.com/Start9Labs/cups-messenger on Star9.
You can run somethibg like this on an old computer, even a raspberry pi if it's all you're running.
Discussion
https://keet.io is also pretty easy. People can be as anonymois as they want to be.
Join me on Keet.io - The Psychedelic Libertarian Repository (no expiry)
pear://keet/yry5f6hctja4dwbmpfxbpngqgjp6oiz3hqtzhimcseumfusi3koaeykkdtpe7u4pkubpkt4tykmjrpknr5an9ncjnw63w711x1q8pmkeqc