Right-click the contact folder you want to lookup, choose Properties and in
the Outlook Address Book tab, choose the Show this folder as an e-mail
Address Book.

Then when you open a new message in the To button, you'll have this contact
folder to choose from.

Cheers,

--
Ricardo Silva
Add-ins for Microsoft(c) Outlook(c)
http://www.rsoutlook.com/us/products.html