It's been years in the making, but the Thunderbird developers have finally brought the open-source email client to Android.
/nosanitize
https://play.google.com/store/apps/details?id=net.thunderbird.android
Note that for end-to-end encryption, you have to have PGP support installed (use OpenKeychain: Easy PGP from the Google Play Store).
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The Thunderbird email client finally landed on Android, and it was worth the wait