I was reading in these days the future update process that Hotmail will have (new interface, automatic inbox refreshing, preview pane etc.) and a thing has attracted my attention: Hotmail will have a "Boomerang"! 
What is this? A new arm to fight spam? 
No... none of this... as explained by one of the author, Omar Shahine, Boomerang is a new idea to fight email spamming and to secure email delivery.
Basically, Boomerang takes advantage of two very simple things: 1) The Message-ID header, 2) A hash, and 3) the In-Reply-To header. As explained by Omar, when you send an email at Hotmail, it creates a Message-ID (hashed) that is unique to you (the hash is a one way hash).
When another mail client receives the message from Hotmail, it contains this special Message-ID and the In-Reply-To message header that allows mail clients to uniquely tie a reply to another email (the one you sent). Each subsequent response to that thread maintains the Message-ID in the form of the References header, so replies to this email are never considered spam and it will be placed in your inbox ensuring that it bypasses any content filtering.
This seems an interesting idea, but saying that it's the end of mail spamming is a terrible error (spammers will be ready for example to generate a fake hash I think) however this "Boomerang" can reduce it a lot I think. I'm curious to see it in action... 