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.
MailMate is not for everyone, but if it’s for you then you might never have to switch email client again.