MSN to can spam with Sender ID
 

In an effort to achieve Bill Gates' promise to end spam by 2006, Microsoft sent a message to several large ISPs saying that any e-mails sent to its Hotmail and MSN services without a Sender ID would automatically be marked as spam. The filtering technology, which has been roundly dismissed by the open-source community, performs a DNS check to ensure that the incoming message comes from a valid mail server.

http://arstechnica.com/news.ars/post/20050623-5036.html