There are plans to add support for it to Tutanota[0], although ironically that blog post was written over a year ago and their FAQ still says that they don't even support interoperable PGP.[1]
When ProtonMail were asked two years ago about supporting Autocrypt, their response was quite negative but also vague[2], and there doesn't seem to have been much progress since.[3]
For those unfamiliar:
https://autocrypt.org/
There are plans to add support for it to Tutanota[0], although ironically that blog post was written over a year ago and their FAQ still says that they don't even support interoperable PGP.[1]
When ProtonMail were asked two years ago about supporting Autocrypt, their response was quite negative but also vague[2], and there doesn't seem to have been much progress since.[3]
[0] https://tutanota.com/blog/posts/email-encryption/
[1] https://tutanota.com/faq/#pgp
[2] https://old.reddit.com/r/ProtonMail/comments/8dqk5n/protonma...
[3] https://github.com/ProtonMail/WebClient/issues/120