[MarsIronPI]

Name one feature (i.e. thing that requires client support, not UI) that's new in email since 2005 and that's also worth having.

[aboardRat4]

Without IMAP NOTIFY email is almost unusable.

https://www.rfc-editor.org/rfc/rfc5465.html

It's from 2009.

Without managesieve (2010, https://www.rfc-editor.org/info/rfc5804/ ), any decent filtering is impossible. (And no, it's not supported by almost any email client)

JMAP has already been mentioned.

I also don't remember any email clients supporting carddav.

>not UI

Why not UI? UI is very important. Even such a basic thing as muting folders works like cr*ap in k9mail. Is it better in "fairemail"?

[bigfatkitten]

JMAP.

Modern authentication.

S/MIME with cryptography that isn’t RSA.