Unibox, a new Messages-style email client that was first announced in August of 2012, is now available for download as part of a public beta test.
The app is designed to make email more "personal" by organizing emails into a contact-centric view. Unibox's interface is divided into two main panes, with the left side displaying contacts and the right side displaying all messages that have been received/exchanged with a particular person or website.
Composing emails with the app is done in a single window, which eliminates separate compose windows utilized by existing apps like Mail. All attachments sent to and received from contacts are readily available, and attached media is presented in a grid view.
Article Link: New Chat-Like OS X Email App 'Unibox' Enters Public Beta