Postbox supports Gravatar for looking up profile and conversation photos. When you read an e-mail message, a hash of the recipient's e-mail address is sent to the Gravatar service to see if that user has created a Gravatar profile picture.
You can configure this behavior by going to:
OS X: Postbox > Preferences > Advanced > Web Services
Windows: Tools > Options > Advanced > Web Services
...and then toggling the option for using the Gravatar service.