Outlook 2013 and distribution list

[twnixon]

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?

From:	postmaster@mail.hotmail.com
Sent:	Sat 1/25/14 4:32 PM
To:	winona.nixon@verizon.net
	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. Show content|I trust postmaster@mail.hotmail.com. Always show content.

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.