roncerr--Thanks for the response. Using WLM 2011 the Check Names icon (with check mark) seems a permanent part of the New Message window's ribbon. It is there right from the start when I open that window, before entering any email address.
I suspect its purpose is along the lines you suggest, but I feel I have entered addresses that are not in my Contacts list (or erroneous versions of ones that are in the Contacts list) and nothing happened. I have never seen the Check Names window that, from Googling, I see can occur. I do not want to see it, since I understand when it appears and you click it off, WML will not permit the email address to be used.