1) Quit Postbox so that it is no longer running.
2) Remove the Caches directory, named "Postbox," located here:
Mac
<your home directory>/Library/Caches/PostboxApp
(Need help finding this folder?)
Delete the folder "Postbox".
Windows
<your home directory>\AppData\Local\PostboxApp
(Need help finding this folder?)
Important: Make sure you are navigating to AppData\Local and NOT AppData\Roaming!
Delete the folder "PostboxApp".
3) Restart Postbox.