I’ve used both. I switched from Fastmail to Proton then back to Fastmail. If you’re just starting on your privacy journey I’d still recommend Proton.

When I switched to Proton they only did email and that’s what I wanted. Instead of focusing on email Proton expanded into other areas like VPNs, Proton Drive, and password managers. I already had good privacy focused solutions for all of those problems, so for me personally I didn’t like where they were spending their development time.

As a Linux desktop user and an iOS mobile user I was often one of the last to have new features available for Protons applications which got to be really annoying.

To use desktop email I had to install Proton bridge which required a GUI to run. It was always having issues. Super frustrating.

I really disliked that Proton didn’t give me a way to use SMTP without going through their bridge. I have three home servers configured with Fastmail app passwords limited to only SMTP to send me notifications for updates and other alerts. This would have been really flaky to make work with Proton.

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.

I also feel as if Fastmail is giving me more for my money. I remember having pain points with Proton and wildcard emails with custom domains and trying to use their hidden email service. All of that is much easier with Fastmail. I also had a few sites not allow Protons masked emails but Fastmail worked fine.

I’d say, if privacy is your main thing and you don’t already have some of the services offered by Proton go with them. If what you’re looking for is as much privacy as email will let you have without using non standard software, and you just really want reliable solid email, Fastmail is the right choice.