Entries in this subkey are used by the Address Book support functions in the Mail program. Most of the entries for this subkey use default values specified by the Mail program, and the Address Book subkey might not be present under the Mail key. The entries in this subkey control the default address directory displayed in the Address Book. Do not change any of these entries if they appear.
This is the Registry path for this subkey:
HKEY_CURRENT_USER\Software\Microsoft\Mail\Address Book