Not sure yet. ProtonMail doesn't work offline either and is apparently not straightforward to use with other clients. I wish it were sensible to host your own email, but I've heard that's a horrible misadventure.
ProtonMail iOS app works offline, at least you can read email and draft email offline. The client is also open source but they don’t seen overly keen on accepting contributions.
Ah, I didn't check iOS - I would use an Android client.