Yes, you read good, I sad both OS X and Windows, something that it’s quite hard to find right now because many apps are built exclusively only on one platform. Postbox represents a truly replace to the integrated Apple Mail app and Windows Live Mail.
Postbox behind its simple and friendly window, has powerful features. First of all, supports Facebook, Twitter, LinkedIn accounts in order to recognize your contacts and show them in the inspector sidebar, with links and social profiles. Not to mention Dropbox integration for sending large files and Evernote to save emails. All of these are ready to be used by setting up an account from the Preferences pane.