Thanks to jerry for the heads up in BPN
According to a post at IETF's MAIL-SIG list by John Levine; Google has begun to sign outgoing email from Gmail with Yahoo's DomainKeys signatures. This is the first large provider of email that is actually doing so (not even Yahoo has started that yet).
DomainKeys is a technology for verifying both the domain of each email sender and the integrity of the messages sent (i.e,. that they were not altered during transit). And, once the domain can be verified, it can be compared to the domain used by the sender in the From: field of the message to detect forgeries. If it's a forgery, then it's spam or fraud, and it can be dropped without impact to the user. If it's not a forgery, then the domain is known, and a persistent reputation profile can be established for that sending domain that can be tied into anti-spam policy systems, shared between service providers, and even exposed to the user.
It should be interesting to see how other providers will react. Gmail helped rework the standard Inbox size with 1GB of storage. Perhaps they'll provide a jump start to other providers to adopt similar efforts to cut down on SPAM.
News source: NetWizard's Blog