I also examined Tuta (in addition to Proton) as replacement for Gmail, and as others have said for Proton, the big no for me was having to deal with other software components to have IMAP or other access options.

I have integrations e.g. for scripts, external mailers and so on, so I don't want to pay something AND still have to setup something myself and maintain it. Fastmail has the concept of app passwords, which are username+password combos that you can scope to specific services (e.g. only sending emails, only accessing calendar, ...), which is perfect for my needs.

I'm in the process of leaving Proton and moving to Fastmail. I'm now paying for Mullvad and Fastmail, but haven't had a chance to port over a bunch of Simplelogin aliases I have over to Fastmail yet (my fault for not using a custom domain for those, which I've now rectified with Fastmail).

Despite having Proton since 2018, I've really gotten fed up with their service quality — they seem to half-ass most of their services and it's a real pita to use their ecosystem. People have been complaining for ages and nothing changes, so I've decided to take my business elsewhere.

Hopefully I don't get flamed for my decision; that's another annoying aspect about Proton. There's a vocal portion of the userbase that are zealots and really take you to task when they feel you are criticizing Proton in any way.

Nowhere near the same - there's no JMAP or true IMAP on Proton.

Running a relatively-small opt-in email list, I've had problems with deliverability to Proton.

I haven't used fastmail, but proton works well for me. It's mildly annoying to have to run a local IMAP proxy to use a proper email client for my mail, but it's not a dealbreaker. And with the US having had over a decade track record of violating everyone's digital privacy, I enjoy the fact that proton isn't a US company and is therefore less susceptible to that pressure.

I have proton email since they had the .ch domain and two passwords to open your inbox, never used it for anything serious. Fastmail works well because you should never think of emails as private or anonymous, the protocol itself wasn’t designed with that in mind, I won’t even mind to have my whole email publicly shared like a blog because I never used it for anything private.

I tried both and found Fastmail vastly easier to use.

The encryption part wasn’t that valuable for me and ofc Proton doesn’t encrypt the metadata and unless they’re proton too it’s not E2E so I’m happy sticking with Signal for that sorta stuff and leaving email for talking to businesses.