With almost 120 million e-mail users on its Hotmail and MSN networks, Microsoft has confirmed that it placed the IP addresses of rivals AOL Time Warner and EarthLink in its "blocklist" which resulted in any e-mail from these services being blocked.
Microsoft use a variety of methods to detect spam, filtering software from San Francisco Company, Brightmail focuses on the message content to detect spam. In this case however, it was Microsoft's additional IP "blocklist" that caused the problem.
With ever increasing levels of spam mail being sent, protections against them are becoming more complex, and it is almost inevitable that some legitimate messages will be incorrectly blocked as unsolicited mail. http://news.com.com/2100-1025-990653.html