It looks like Hotmail done gone and blowed up again.
I am seeing multiple reports that mail server connection attempts to hotmail.com / outlook.com recipients are either timing out or resulting in mail to legitimate users is being rejected with a "554 Transaction Failed" error. This appears to be affecting at least one large email service provider, and probably others.
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.