After a long journey from Apple Mail and Airmail, trying out Spark and Postbox, I found MailMate at last. It works fine with all my accounts (including Exchange via IMAP) and is so much more reliable than the others! And it has so many wonderful features like the several layouts, correspondence view, fast access via menubar icon ( listing unread/flagged mails with quick actions), custom keybindings, Markdown in message composer, SpamSieve integration, great search capabilities (filters, speed, Spotlight support), robust offline support, and on and on.
The only thing I dislike is the message list view; it's a 2000er-style simple list that a lot of other mail client do better, e.g. Airmail and Spark. I miss swipe gestures and breaking down a message in the list into 2+ rows and showing an avatar to the left. I improved MailMate a bit for me by assigning touch gestures via BetterTouchTool and by setting the line height delta to 10 (via font settings), so that the list is much readable.
If you are fine with IMAP only, then Mailmate is a no-brainer and will make you happy. Cudos to the developer!!