How can Microsoft legally read a blogger’s hotmail emails to find the source of a leak?

Microsoft apparently went through the hotmail account of the blogger who posted leaked Windows code to find the source of the leak. Can they just do this? Is it legal for them to just go through and read peoples’ emails whenever they have what they think is a good reason to do so?

Story added 21. March 2014, content source with full text you can find at link above.