Postbox 3 now supports Gravatar for looking up profile and conversation photos. When you read an e-mail message, a hash of the recipients e-mail address is sent to the Gravatar service to see if that user has established a Gravatar profile picture.

You can configure this behavior by going to:

Mac: Postbox | Preferences | Advanced | Web Services

Windows: Tools | Options | Advanced | Web Services

and toggling the option for using the Gravatar service.