Can you only send encrypted messages to recipients who also use Google? If it isn't S/MIME then as far as I'm concerned it almost isn't email. Anyone can alread

Can you only send encrypted messages to recipients who also use Google? If it isn't S/MIME then as far as I'm concerned it almost isn't email. Anyone can already make encrypted ZIP files or other proprietary encrypted attachments and send them via email. If it's not S/MIME, it feels like just a convenience layer on top of that.