I'm really enjoying #FairEmail app on my phone. It have been the smoothest SMTP/IMAP experience I've had on any operating system. Even Thunderbird is not as smooth and as customizable. I've been actively using it for some months now, and frankly, I'm impressed. The UI design takes few hours to get used to, but after that and with some personalization, it would look as trivial as an email client can get.
It is #OpenSource (GPLv3) btw: