It’s maintained, zanghai updates dependencies every week: https://github.com/zhanghai/MaterialFiles/commits/master

Just no new releases. I haven’t found any bugs and it has all the features I need, so I don’t miss anything. When an app is finished, you don’t need new releases every week.

Yes, but for me standards are more important than encryption. I can encrypt mails with pgp, than they are end to end encrypted, imap doesn’t matter. I considered proton when I switched to mailbox, but usually I don’t send encrypted mails, because the reciepents cant read them, so I wouldn’t use the pros of proton.

Why do you need a mobile app? It’s just email, all IMAP clients should work.

I have a mailbox address since years, never use the webui for checking mails, only for changing some settings, I access my mails with Thunderbird on desktop, Fairemail on Android. Both apps have builtin PGP, so you shouldn’t care what the provider supports.

The spam filter in mailbox is glorious, never got a spam there.