Confronted to a question about Microsoft's lack of a good antispam solution, Vinny Gullotto, head of all things malware with Microsoft, went to describe what he calls "Zero-Day Spam" as a way to fight the spam epidemic. The term was already briefly mentioned by Trend Micro earlier.

Gullotto's vision is to use incoming emails as the starting point of a real-time investigation to follow the malware path. He justified this by explaining how each malicious spam has someting in common : it starts a chain of events having for final objective to steal or exploit something. 

The idea is to go from one hop to the other starting with data found in the email, and check at each stop if it related to malicious activity. This could lead to identify unknown spam using new stealthy techniques by uncovering their ties to known malicious activities (exploit droping websites, offshore prescription drug selling sites, fake Rolex boutiques, etc... for example)

Of course, this is not much different from what Websense / Surfcontrol is doing, and it does not take in account the Pump & Dump kind of spam. But the term is catchy.