At first thought, you'd think: All that would be needed to make this happen would be for gmail, outlook mail, and Apple's clients to support signed and/or encry

At first thought, you'd think: All that would be needed to make this happen would be for gmail, outlook mail, and Apple's clients to support signed and/or encrypted mail and display a logo if the digital signature matches, say, a signature in your contacts. But spammers would probably just throw a "signed" or "verified" logo or text at the bottom of the mail and fool or confuse most people into trusting an email they shouldn't trust.