possibly offtopic is best or services discussion, or even possibly this board is perfect to post it.
here are what comes to mind:
1. protonmail.com
open source, based on switzerland and encrypted. visit the site for more information
this is the easy option because it is the regular email with privacy.
2. bitmessage.org
i am not familiar with this myself. i just know it exists. it is decentalized, encrypted and peer-to-peer. it seems secure and anonymous. but check it yourself
this is the hard option because the other person has to know how to use it.
https://en.wikipedia.org/wiki/Bitmessage