I've just found this:
*Block a Sender in Windows Live Mail
To add a sender to your list of blocked senders in Windows Live Mail or Windows Mail: Highlight a message from the undesired sender in the message list. Make sure the Home ribbon is active. Click the down arrow beneath Junk in the Home ribbon.
Select Add sender to blocked sender list from the menu.
Select Add sender's domain to blocked sender list from the menu to block messages not only from this sender but from all addresses at the same domain (what follows the '@' sign).*
Hope this helps explain it?