you will need to add a lookup recordset to the page to check for the email address validity.
Create a recordset that will filter the email address column on the emailAddress form element.
the recordset will create a variable named $totalRows_<Recordsetname> (where <Recordsetname> is the name of the recordset) that contains the number of rows returned.
Use server validations to validate that the recordset returns a row.
Set the evalidation type to Numeric, set the server variable to $totalRows_<Recordsetname>.
Set the minimum number to 1 and the maximum to 1000.
you can then use the validation show if behavior to display your error message.