I found https://email.faircode.eu/ while searching for an alternative to the stalwart K9 Mail on the F-Droid store.
My first impressions are very favorable. It’s nice and easy to use and packed with features.
For example, I couldn’t get folders to display properly with K9 and FairEmail does it with ease.
It’s open source but I went ahead and donated to the developer to get access to the pro version.
I think it might be my new default mail client.