Last year, Microsoft acquired the hosted (or managed) email security service provider, Frontbridge. For some time, I've been saying that there's an interesting competitive advantage enjoyed by Frontbridge and other services providers, such as Postini, MX Logic, MessageLabs, and BlackSpider.
Companies that offer a service to a large number of customers get to see a lot of spam and other unwanted email. This is very useful to spot new types of spam campaigns and spot them quickly. As spammers shift to a botnet model, spam campaigns are hitting harder, sending more messages over a shorter period of time. The quicker a spam control solution can notice a new campaign and block it, the less spam is actually received by users.Microsoft says that "92% of all email received at microsoft.com is spam," so the FrontBridge team is now receiving an enormous corpus of new spam, which should help them to be more reactive to new spam campaigns..
Interestingly, Microsoft claims that it's also extracting reputation data from spam sent to hotmail.com and msn.com, but I'm told by my buddies at Symantec that this is still protected by BrightMail technology.
[edited for clarity June 15 4pm BST]