If you use Fastmail and wish they offered a Mac app, check out FMail2, a labor of love from a developer in France. It’s free (but buy him a coffee to show your appreciation).
Email provider Fastmail has native iOS and Android clients. But nothing for macOS. The web interface used by Fastmail is excellent. The big thing missing, is that the web interface cannot behave like a native mail application. It can, for example, not handle email links. When you click on such a link it is the macOS Mail application that opens. That is not what we want.