Unibox is an email client, in the form of an AIM client. The source list is defined by author, not by individual messages. As noted in this article by Arnold Kim on Unibox, this means Twitter and Facebook spam is limited to just one item in your inbox. Similarly, short IM-like conversations don’t clutter an inbox with multiple messages in your inbox — just one.
It’s like Messages for Mac, but the data source is your email. Threaded messaging taken further. Very clever.