ISP signatures on outgoing mail

Anyone heard of a proposal for ISPs to automatically sign outgoing mail headers? Problem has been that spammers send email by one path but forge a reply-to or from address at another location. Most recent case is hotmail, which got blocked by netcom over a spam attack. Actually mail didn't come from hotmail, was forged to look like it came from there. Same thing has happened to remailers. They need a standard for which headers to sign, then a dig sig can be included in the headers to check that a message came from where it claims.