Fastmail is a webapp, and it's very obvious. I use it out of laziness, because I don't use email on my phone that much. I wish they'd make a proper mobile client, but I don't think they care, since it's been like this forever.

OT1H, I don't have a horse in the fight about whether you're right or not, but I was curious because if it is a web app they did a damn stellar job

But no, it's a formal android app and I offer two pieces of supporting evidence: the resource browser shows the apk very obviously has a bazillion android resource files <https://imgur.com/a/pUbQ8Bp> and it supports native fingerprint auth

I'm not sure why it doesn't work offline, then. The UI refuses to load at all if I don't have an internet connection.

Native fingerprint auth can be implemented on top of a webapp; that isn't evidence either way.

Native app doesn’t guarantee “caches data for offline access”.

True. I seem to remember that the UI itself did not load when there's no internet connection, but I just tried it, and it does now. So maybe things have changed, or I misremembered.

Either way, no offline access in an email app... pretty bad.

It's not a native app. The main interface where you view your mailboxes, compose new emails, edit settings etc is a web view. The interface doesn't even load if you're offline, let alone your emails. Certainly some parts of the UI are native, but that's true of any web app.

I dunno if it's native or not, but the Fastmail app doesn't work when you're offline.