With Fastmail everything uses open standards, IMAP, SMTP, CalDav, CardDav, and WebDav. It all integrates really well with my laptop and phone without any special tools. I end up using those services much more now. The downside to these open standards is you don’t get end to end encryption that Proton offers.

Yes, this openness of Fastmail makes them a really good provider unlike Proton that is always pushing for more proprietary garbage and uses “encryption” and an excuse for everything.