First, you need to create the recordset to filter the email address column on the email address form element.
the recordset will create a variable named:
$totalRows_RecordsetName
(where RecordsetName is the name of the recordset) with the number of records returned.
the idea is to use numeric validation to make sure that recordset does not return any records (is empty)
add server validation, chose numeric validation and set the server variable to:
$totalRows_RecordsetName
Set the minimum number to -1 and the maximum number to 0.
to show the error message, use the validation show if behavior.
