What open standard (that are not used by proton too)? If anything proton uses GPG while tuta uses a custom system (which is why they also encrypt the subject).

Proton does PGP for you. There are countless standards compliant mail services where you could use your own email client and do your own PGP -Thunderbird is probably the easiest but It’s a trade-off.

Tuta also locks you down with their lack of IMAP support. You can only use their client. At least Proton has a bridge and allows you to use your own GPG key.

Anti Commercial-AI license