I recently wrote a step by step article about this, for people who might not know how, such as family members, et al. I chose ProtonMail (despite also being a FastMail customer) because I’m a bit concerned about the new Australian encryption key escrow mandate (which I assume affects FastMail) and I like ProtonMail’s “don’t store plaintext” approach, even if it does need special client software.
If you’re emailing other people on ProtonMail, it fetches their keys from the (presumed trustworthy) server, and does end-to-end encryption.
In that mode it’d at least as secure as iMessage (before Apple backdoored it by adding automatic key and plaintext escrow).
Most emails use TLS, so they’re encrypted on the wire between servers.
ProtonMail then encrypts the plaintext as soon as they receive it, for storage. It stays encrypted from that point until it reaches the client.
For most mail it’s not e2e, but it does cut down on the opportunities for the mail to get seized by anyone who can compel the provider to turn over their records.
https://sneak.berlin/20201029/stop-emailing-like-a-rube/