Looking at https://www.guidingtech.com/k9-mail-app-alternatives-android/ I find the curiously named Tutanota.
Weird name but awesome email client and one of the few names in the market that can give ProtonMail a run for its money. Another open-source email client with a presence on both Android and iOS, Tutanota also uses end-to-end encryption but misses out on OpenPGP.
That allows them to encrypt the subject line (unlike ProtonMail), but to read the encrypted message, the recipient will have to click on a link and visit Tutanota website and enter a password.
One additional feature is 2FA. which secures your mobile and desktop clients. Yes, Tutanota also has native clients for Windows and macOS. Based out of Germany, which is governed by GDPR, Tutanota offers more space in the free plan, and their paid plans begin at €12/year.
Tutanota’s UI is better than K-9 Mail but less attractive than ProtonMail. There is support for swipes which is configurable, but no snooze or self-destructing emails yet
I am not quite certain about this (because it uses its own servers, although all email is encrypted), but at least it offers clients for both Android & Windows, so I will post this in case it helps someone else.
More possibilities - all Windows & Android, free; I need to investigate these:
