'We support the establishment of an independent trust authority or authorities around the globe that could spearhead industry best practices, and then serve as an ongoing resource for email certification and customer dispute resolution. In short, these authorities could provide mechanisms to identify legitimate email, making it easier for consumers and businesses to distinguish wanted mail from unwanted mail. Of course, any technology designed to establish the identity of legitimate commercial firms and associate them with a trusted sender 'seal' should be based on open standards and developed with broad input from affected industries.'Gates Sends Letter on Spam to Congress - May 21, 2003 - InternetNews.com.
See the Trusted Email Open Standard which is stored as a PDF here at the FTC and described here. The standard came out of several years of research and was formally presented to the FTC in April of 2003.