Postbox 3
Postbox has released version 3 of its eponymous email client, adding numerous Gmail, social networking, and integration features. The Thunderbird-based Postbox 3 contains a large number of new features, such as support for Gmail labels, a new send-and-archive function, support for Gmail keyboard shortcuts, and the capability to detect dates automatically and convert them to Google Calendar events. Postbox has also gained several social networking capabilities, such as displaying profile photos from Facebook, LinkedIn, and Gravatar, and posting to Facebook, Twitter, and LinkedIn. A new favorites bar provides fast access to preferred accounts and
folders, and better integration with external apps and services like Evernote, Dropbox, iCal, and Growl increases productivity. Finally, Postbox can now create canned responses based on custom templates, and it boasts better overall integration with Mac OS X, including support for gestures and the new full-screen mode in Lion. ($29.95 new, free for users who purchased after 15 August 2011, $9.95 upgrade, 21.4 MB)
I am not sure why PostBox gets to be so strongly linked to Gmail in this article? It is a strong client based on Thunderbird and hence ready for any mail service
You're right, and I've addressed that in my edit pass. Postbox added a lot of Gmail-specific features, but it's certainly not restricted to Gmail in any way.
It's still a disappointment in one way that's critical to me: Address Book filtering by group. Mail can check to see if each incoming email's sender is a member of a group, and then send any that match off into a read-later folder. That's how I handle mailing lists. Apparently, there's no equivalent in Postbox.