Comment Re:A large part of the solution already exists (Score 1) 222
DomainKeys have nothing to do with MUA, but at the moment they are the only way I (the mail recipient) can authenticate a PayPal e-mail. Since DomainKeys are not part of SMTP and since - other than a verbal/written PayPal request - there is no requirement for SMTP servers to discard unsigned e-mails, I can't trust that a 'PayPal' e-mail that reaches me must be authentic. You are happy to oblige PayPal's request, and that's good for everyone, but not everyone is going to follow your example.
