The networking or routing codes report status about the delivery system itself. These system components include any necessary infrastructure such as directory and routing services. Network issues are assumed to be under the control of the destination or intermediate system administrator.
- x.4.0
- Other or undefined network or routing status
- x.4.1
- No answer from host
- x.4.2
- Bad connection
- x.4.3
- Directory server failure
- x.4.4
- Unable to route
- x.4.5
- Mail system congestion
- x.4.6
- Routing loop detected
- x.4.7
- Delivery time expired