I am evaluating Runbox and Migadu right now; My only nitpick with Runbox is their pricing page, they need to clean it up and simplify it. Its information overload. I can always ask/click for more details but present me with the most basic info up front and centre.
The services are good and when needed support is also good. Clients have given good feedback.
In the past i have recommended protonmail and fastmail.