If you own the latest version of securityassist we added a server validation for this purpose.
If you don't have the latest version you can still use the old server validation to achieve that result you just have to create a recordset that is filtered by the entered email address and then add validation to make sure that recordset doesn't return a row... which is number validation that the variable: $totalRows_RecordsetName has a value of zero.
You should be able to find several examples of us walking people through this process by searching this forum.