Ask HN: How did Microsoft email services became the primary source for spam?

Just did monthly spam folder sweep. Sometimes legit emails end up in spam, so I regularly make sure that something important was not lost amongst trash. Looking at sender details I noticed that 99% of spam emails are now coming from Microsoft email services, like Outlook and Hotmail. What happened? Did other email providers improve security? Is there some security hole in Microsoft services, that allows easy registration for throwaway email accounts? I'm interested from purely technical aspect what might shifted most spam to MS services?

And most important question: is Microsoft going to do something about it, as service abuse reports seem to make no difference.