It’s not. Proton’s “security” is basically pointless and induces a huge hassle if you’d like to use anything else than the web client.
As you said, both Proton and any other mail client send mail in plain text over SMTP unless encrypted using PGP/GPG.
Fastmail is just a much nicer email provider IMO, and I can consume emails / calendar / files using third party clients. There’s also Tuta and other mail clients, I’m just warning you to maybe steer clear of Proton unless you intend to use Proton-specific features.
Ideally, I’d pick an email provider with data sovereignty in Canada, but short of self-hosting (which isn’t a great idea with email), there are basically no decent options.
PlexSheep@infosec.pub 3 days ago
IMAP and SMTP, the protocols mainly used for emails besides whatever weird shit Microsoft is doing, nowadays all have variants going through a TLS encrypted session like HTTPS.
That doesn’t change the fact that email is not up to the task of modern secure communication (TLS is not end to end encryption for example, and smime and pgp are super user unfriendly and have their own weirdnesses), but makes it better at least.