MailMate is an ambitious powerful email client for Mac OS X. It is designed for and only supports IMAP, but it also works in full when offline. The aim of MailMate is to make navigating, viewing, replying to, and searching for emails as easy as possible. This is done by providing keyboard control for most actions, by having a strong focus on eliminating the need for any repetitive tasks, and by optimizing any time-consuming tasks.

Features:

• Multiple Accounts: Universal mailboxes are supported including a unified Inbox. All IMAP servers are supported including SSL/TLS and server certificate validation.
• 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.
• Keyboard Centric: Navigate, read, and reply to emails using the keyboard only. Change current mailbox or move emails to a mailbox using abbreviations.
• Layouts / Views: Alternative window layouts are available including a widescreen layout. Special views are used to show thread arcs and email statistics.
• 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 Growl notifications. Specify format strings for menu bar menu items and Growl notifications.
• 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.

Limitations: 30-day trial.

Requirements:
OS X 10.10+
64-bit Intel

Also available are:
Version 1.12.5 for OS X 10.8-10.9
Version 1.9.6 for OS X 10.7
Version 1.7.2 for OS X 10.6
Version 1.4.2 for OS X 10.5