I’m with @MacSparky on this. I’ve tried using 3rd party mail clients, especially Airmail and Spark but I always run into a problem. My problems are not syncing or instability but incompatibilities. For example, in Airmail, my HTML signature does not show up properly in all instances whereas in Apple Mail, it does. In Spark, I can’t send an email via collaboration/sharing in Pages or Keynote. The email is sent but not the link for sharing/collaborating on a Pages or Keynote document. In Apple Mail, it works perfectly.
I guess I’m relegated to Apple Mail. It works fine but I’d love to have the advanced features of Airmail or Spark.