have been using FairEmail and it works really well. I have to admit it is a very verbose client which offers a vers large number of options and settings even as you just want to send an email (there’s a popup with how you want to send it) but it has never failed me and has a very good support for updates and guide for setting it up. It’s free but if you like it I highly recommend to support the developer who is fully committed to it (not personally related to the project in any way). I would be very upset if this project were to die and I had to find another client.

Edit: link https://github.com/M66B/FairEmail

I’m a big fan and enjoy it but there’s a few things I don’t like. That said, K9-mail is currently missing simple features like always remember for simply showing images from this sender and that’s annoying. But I’m really looking forward to synchronicity between clients, which Thunderbird will have out the box and FairEmail doesn’t.