Gravatar Profile Pictures

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.