Shortlist of features:
- Universal mailboxes: One Inbox for all of your IMAP accounts.
- Extensive keyboard control: Switch mailboxes more...
- Integration with Calendar/iCal, BusyCal, and Fantastical.
- Integration with Evernote, Things, and The Hit List.
- Support for SmartyPants when using Markdown with HTML generation.
- Drag'n'drop email/mbox files to a mailbox to trigger importing.
- OS X 10.6 or later
- IMAP email accounts