Correo is an open source mail client that mixes Mozilla and Camino technology.
Correo is to Thunderbird what Camino is to Firefox. Built with Mozilla code and fitted with a Camino like interface, Correo is a welcome addition to the mail client scene. Even though the application is still only at 0.3, its features look promising. The interface has the buttons and design of Camino and a multi column interface.
Correo is already Leopard compatible. It can integrate the Mac Address Book and, like Camino, supports Keychain, so you can use your certificates to encrypt your emails. You can add attachments to any message, handle multiple accounts, whether POP or IMAP and open up messages in a new window. Correo is enjoyable to use and should make for a good alternative to Apple Mail and Mozilla Thunderbird in the future.
It's obvious that the application isn't ready yet though. Correo is pretty unstable and can crash for no reason. IMAP support also seems to have some issues every now and then.
A promising open source mail client, with Mozilla and Camino technology, Correo is still too unstable for any serious use.