The (&(objectClass=user)(mail=)) search filter is invalid.
<detail><ExceptionDetail xmlns="http://schemas.datacontract.org/2004/07/System.ServiceModel" xmlns:i="http://www.w3.org/2001/XMLSchema-instance"><HelpLink i:nil="true"/><InnerException i:nil="true"/><Message>The (&(objectClass=user)(mail=)) search filter is invalid.</Message><StackTrace>   at System.DirectoryServices.SearchResultCollection.ResultsEnumerator.MoveNext()
   at System.DirectoryServices.DirectorySearcher.FindOne()
   at SP.HR.EmployeeService.getUserADInfo(String filterCriteria)
  at SP.HR.EmployeeService.GetEmployeeByEmail(String email)
   at SyncInvokeGetEmployeeByEmail(Object , Object[] , Object[] )
   at System.ServiceModel.Dispatcher.SyncMethodInvoker.Invoke(Object instance, Object[] inputs, Object[]& outputs)
   at System.ServiceModel.Dispatcher.DispatchOperationRuntime.InvokeBegin(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage5(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage4(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.MessageRpc.Process(Boolean isOperationContextSet)</StackTrace><Type>System.ArgumentException</Type></ExceptionDetail></detail>