Here is a list of steps for troubleshooting when email server being down for maintenance:

1. Check the system log to see the cause of the outage.
2. Check the infrastructure components, such as the network, hardware, software, and configuration settings.
3. Identify any known or suspected vulnerabilities and patch them.
4. Monitor the system to ensure the outage is resolved.
5. Test emails after the repairs are completed to ensure everything is working properly.