What's the deal with delivery to Hotmail/MSN???

Being the largest online news source in the Western Pacific, we've got a pretty sizable distribution list for our daily e-mail news headlines. I was concerned this morning when I noticed that delivery of last night's newsletter failed delivery to everyone with a Hotmail and MSN e-mail address.

I tried checking the usual sources for info on this, but nothing yet. Hopefully, it works itself out.