I’ve used Postbox for a number of years and have found it to be the best overall client. I got a lifetime license for $49 which i think was a bargain. I spent time using Sparkmail, Airmail Mailmate and Mail.app and find Postbox has done the best job of striking the right balance for a full featured mail app. As noted in other posts there is no iOS version but I generally only us iOS to triage mail.
I’m surprised and a bit disappointed David and Stephen didn’t include it in their recent review of email apps.