All but the last are used by everyone as it is basically impossible for mail to be accepted reliably from an email server that doesn't have those set up correct

All but the last are used by everyone as it is basically impossible for mail to be accepted reliably from an email server that doesn't have those set up correctly. As for S/MIME, certificates are a pain to deal with for most people and all they really care about is transport-level, not E2E encryption.