Windows Mail will show both the contact name & the
contact email address.  You can't change that.
If you send to multiple recipients, and don't want them to see each other's
email address, use the BCC: field.
If you don't see the BCC field, in the compose window, click on View - Show
All Headers.

If you used to put multiple email addresses in the To: or CC: fields in any
other email program, the recipients were able to see each other's address,
whether you saw it or not.

Windows Live Mail only shows the contact name, unless the contact has more
than one email address.