Now that Microsoft is to deploy SPF for Hotmail users it looks like there is some sort of standard for checking authenticity of mail senders around.
More information is available
Nice thing about this standard – it uses the DNS system which has a concept of authority in terms of a final arbiter as to which Domain Name links up to which IP address. The same system already says which server accepts mail for which domain name, so this is completing the loop to say this server can send mail for this domain (why isn’t there an XM DNS type?).