Yesterday’s announcement that Microsoft is going to merge its nascent Caller ID for Email authentication standard with the more populist Sender Policy Framework (SPF) is an interesting development in the war on spam. But what does it really mean? It means that sender authentication is headed towards a standard. Where once there were three, now there are two (Yahoo Domain Keys is another standard, although it’s still a little unclear whether it’s competitive or complementary). Authentication is an important component of the war on spam because it allows ISPs and other email receiveing servers to verify that the sender of the email is who he says he is. Spammers don’t do that. But authentication is only one facet to the…