Add all the fields that you've ever used on that form back to a custom page and check the properties of each resulting control.
Bottom line: Always check the validation *before* you remove the control from the form and, for best results, don't use the simple check box validation. Provide a formula and validation message.
--
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers