The Fastmail app itself is mostly a wrapper around the web app with integrations for notifications etc. Sans notifications it works perfectly as an installed PWA on Android. Ive been using it like that for months.

Alternatively there are lots of IMAP apps available. I was testing Thunderbird for Android recently and that works pretty well too.

Disclaimer: I work for Fastmail. But any opinions I have on here are my own.

ahh aurora store is perfectly fine. I use it with an anonymous session. I think there should be sites that let you download the APK directly, but you wouldn't get updates.

It's probably fine to use one of these methods, just don't forget to update. 😁