Aside: Fastmail was the best email provider I ever used. The interface was intuitive and responsive, both on mobile and web. They have extensive documentation for everything. I was able to set up a custom domain and and a catch-all email address in a few minutes. Customer support is great, too. I emailed them about an issue and they responded within the hour (turns out it was my fault). I feel like it's a really mature product/company and they really know what they're doing, and have a plan for where they're going.
I ended up switching to Protonmail, because of privacy (Fastmail is within the Five Eyes (Australia)), which is the only thing I really like about Protonmail. But I'm considering switching back to Fastmail, because I liked it so much.
Their Android client has been less than stellar in the past but recent releases are significantly improved. Uploading files, in particular, was a crapshoot.
I also chose Proton for the same reason. It hurts that their product development is glacial but that's a crucial component that I don't understand why Fastmail doesn't try to offer.
I ended up switching to Protonmail, because of privacy (Fastmail is within the Five Eyes (Australia)), which is the only thing I really like about Protonmail. But I'm considering switching back to Fastmail, because I liked it so much.