This includes Postbox now running in 64-bit mode along with a new theme, interface and icon set that does much to make it look similar to the built-in Mail app, but retain Postbox's distinct style. Double-row vertical thread view is now supported along with a streamlined message view. In addition to all of this is a major price reduction. When Postbox 2 was released, it was US$39.95. It's now $19.95 with upgrades from Postbox 1 being $14.95.
Postbox 2.5 requires OS X 10.6 and higher and is currently available through Postbox's site. The 2.5 version will be available via the Mac App Store soon.