in the case of a registration form, you would want to validate that the email has not been registered before.
To make sure the email has not been used before, you would need to create a recordset to filter the email column on the email form element from the form collection ion the bindings panel.
name the recordset 'emailLookup'.
In validation toolkit, use number validation, set the server variable to:
Click the plus button, and in the number validation settings, set the minimum number to -1 and the max to 0. this will validate that the emailLookup recordset is empty.