Outlook 2013 and distribution list

I have a distribution list with 50 emails (yahoo, hotmail, comcast, etc).  It was working fine but now I get an error message that some did not go thru using a Hotmail account.  I have verizon.net.  I tried deleting the hotmail addresses from the list.  I still get error saying that some were not delivered.  Below is the error message.  Why is it trying to use hotmail.  I don't have a problem sending any other mail.  Can you assist?

Picture of postmaster@mail.hotmail.com
Sent:Sat 1/25/14 4:32 PM
Attachments: 1 attachment | Download all as zip (19.5 KB)
 FW DC Summer Youth Programs.eml (7.1 KB)
Parts of this message have been blocked for your safety.
This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.