Can you be more specific?
Are you having trouble with the default address that Mail.app uses when you send mail (ie, YOUR email address) or are you talking about the recipient's address when you type someone's name into the "To:" field?
In my experience, when I send an email TO someone and fill their name out in the "To:" field, Mail defaults to the first email address listed in that recipient's Address Book entry. It also defaults MY address (the sending address) to the account designated as the default account in Mail.app's preferences.