Word 2007 sending mail tries to use Exchange address book

Word 2007 sending mail tries to use Exchange address book

My son has Office 2007 installed on Windows XP, although not Outlook - we use Thunderbird for email. In Word 2007 when he does File / Send / E-mail, he gets an error saying that Word cannot contact an Exchange server, and then it doesn't open Thunderbird. Thunderbird is correctly set in the Internet Options as the mail client, so why is Word trying to contact an Exchange server?