What about the email addresses?
Basically, there is no "Activities folder." When you display a particular activities group for a contact, Outlook launches a search of the folders in that group for items that match the contact. For mail messages, the matching is done on the email address. If you change the address listed in the contact, you won't match items with the contact's earlier address.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers