It looks like Hotmail done gone and blowed up again.

For some folks, it has been happening since before noon central time on Wednesday. I personally just started to see it on my own mail server around 5:00 pm on Wednesday. As of this writing (6:30 pm), it's still happening.
This is what makes it tricky for an ESP's bounce handling. You can't just assume that any old hard bounce means that a recipient must be invalid, when an ISP could (and occasionally does) fall down, go boom.
Update: This issue seems to have been resolved sometime after 7:00 pm central time on Wednesday, April 6th.