Cause of the bounce back and email not being delivered

Hello,

I have an issue with MSN email.
I send and receive emails on a normal basis to various email recepients. However, there is one email address I send emails to that bounces back as undeliverable.
There is a Delivery Failure Notification from the Postmaster at Hotmail.com. The email address to the recepient is correct... I double checked. My question is.. What can be the cause of the bounce back and email not being delivered to the email address?

Thank you for a response!