MailMate is an IMAP email client for Mac OS X 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 (OpenPGP and S/MIME), tagging, multiple notification methods, alternative message viewer layouts including a widescreen layout, and much more. A quick overview is given below, but an extensive list of screenshots is also available. You can read about MailMate and its developer here.
Keyboard Centric
Navigate, read, tag, and reply to emails using the keyboard only. Change current mailbox or move emails to a mailbox using abbreviations. Create your own custom keybindings for common tasks.
Markdown Support
Integrated support for Markdown, an easy-to-learn plain text markup language. Create rich text emails using plain text only. Make outlines, tables, code segments, or simply emphasize text. Read more.
State-of-the-Art Searching
Search rapidly in the content of any email header in all mailboxes of all accounts. Search for unquoted or quoted body text. Browser-inspired searching including going back in history.
Smart Mailboxes
Base smart mailboxes on any/all combinations of other (smart) mailboxes and use any/all combinations of matching conditions. Note the advanced and powerful “is in” comparison method.
Dynamic Signatures
The default signature and placement (top/bottom) of the signature in a new message is automatically derived from emails to the same recipient. Use a shortcut to select a different signature. Read more.
Notifications
Optionally display multiple dock counters, multiple menu bar counters, and notifications. Specify format strings for menu bar menu items and notifications.
Layouts / Views
Alternative window layouts are available including a widescreen layout. Special views are used to show thread arcs and email statistics.
Multiple Accounts
Universal mailboxes are supported including a unified Inbox. All IMAP servers are supported including SSL/TLS and server certificate validation.
Full Offline Access
Almost all functionality is available when offline including creating/deleting IMAP mailboxes. Changes done while offline are automatically synchronized when going online.