There aren't many services that support e2e encryption though. I've been a Protonmail user for years and only remember being able to send encrypted emails to other PM users, maybe Fastmail also supports the same protocols?

Regardless, if I'm not mistaken encryption protocols are layered on and not part of the official email spec. I'm not 100% on that so may it was added to the spec as an optional feature, but by no means is it common (especially with the market share Gmail owns).