These are significant changes for the platform as the Sender ID solution has traditionally been championed by the Microsoft team while many other ISPs and webmail providers opted for SPF as their email authentication solutions.
Why has Hotmail shifted away from Sender ID?
While I have not talked to anyone at Hotmail I would say there is a strong corolation to their implementation of DMARC by the Microsoft email team and the relationship it has to DKIM and SPF, it makes sense to move away from Sender ID as the key technology for authentication.
Who else is using DMARC?
I am currently receiving reports from the following ISPs: 126.com, 163.com, AOL, Google, Hotmail, Yahoo, xs4all and Facebook. This also shows the large footprint that DKIM and SPF have achieved with the largest Webmail providers in the world, representing billions of users world wide.
Where can I get more information on DMARC?
Start with reading DMARC.org as a general overview of the solution and how your organization can utilize this Email Authentication solution.