MailMate is an IMAP e-mail client for Mac featuring extensive keyboard control, Markdown supported email composition, advanced search conditions and drill-down search links, equally advanced smart mailboxes, automatic signature handling, cryptographic encryption/signing (https://alternativeto.net/software/openpgp/ and S/MIME), tagging, multiple notification methods, alternative message viewer layouts including a widescreen layout, and much more.
If you're looking for a powerful alternative to Apple Mail, on the Mac then this might be it.
This mail client has some excellent feature, like Markdown support in mail messages and really powerful smart mailboxes.
You might think it's a tad expensive, but I'd recommend trying it out. I did, and then ended up buying it!