This happens when choosing the pop up message and email option for delivery notification while sending fax. This has been handled properly on the newer version.(A message is shown if this is not configured).
This is because the delivery notification feature is not configured on the server.
To set this feature on the server:
1.Start Server Management console.
2.Under Standard Management, Right Click Fax (Local) to open the properties window.
3.Click the Receipts Tab.
4.Ensure that the Enable message box as receipts is ticked.
5.Ensure that the Enable SMTP e-mail receipts delivery is ticked.