Shortlist of features:
- Universal mailboxes: One Inbox for all of your IMAP accounts.
- Extensive keyboard control: Switch mailboxes more...
- Redirection of a messages using a special composer window which only allows editing a fixed set of headers. A $Redirected flag is added to any redirected messages.
- Support for using an external editor to write emails (TextMate, Sublime Text, MacVim).
- HTML signatures can be configured in the Signature preferences pane. They work in both plain text and Markdown more...
- OS X 10.6 or later
- IMAP email accounts