MailMate is an IMAP email client for macOS featuring extensive keyboard control, Markdown integrated email composition, advanced search conditions and drill-down search links, equally advanced smart mailboxes, automatic signature handling, cryptographic encryption/signing (OpenPGP and S/MIME), tagging, multiple notification methods, alternative message viewer layouts including a widescreen layout, flexible integration with third party applications, and much more.

You can read about MailMate and its developer here.