Exchange Transport Service stops working

To resolve the issue you need to:

1. Stop the Microsoft Exchange Transport service.
2. Rename the folder called 'data' to 'dataOLD' under the location C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles
3. Start the Transport service
4. Make sure a new folder called data has been created

You should now be able to telnet to port 25 internally and externally and mail will start to flow again.