This is strange!!
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Thank you for contacting Microsoft Customer Service.
I understand that you would like to know why Microsoft Security Response Center Blog is not hosted on a Microsoft server though it is hosted by ORCS Web.
I would like you to know that ORCS Web is a Microsoft Gold Certified Partner with a certified support team and multiple Microsoft MVPs on staff. Hence Microsoft has given them the hosting rights.
I hope the information is helpful.
Thank you,
Microsoft Customer Service Representative