K-9 Mail
https://f-droid.org/en/packages/com.fsck.k9/
> Does it have good support for plain text email.
Yes.
https://f-droid.org/en/packages/com.fsck.k9/
> Does it have good support for plain text email.
Yes.
https://forum.k9mail.app/t/k-9-mail-is-not-sending-format-fl...