When we try "Send a test email" in an air-gap, the message is:

> The test failed: Exception while attempting to send test email. Message: java.net.ConnectException: Connection refused (Connection refused)

It might be more clear to state the message if it is "Connection refused" something like:

> The test failed: Exception while attempting to send test email. We are unable to connect to insights.cloudbees.com to complete the sending of the test email. Refer to https://docs.cloudbees.com/docs/admin-resources/latest/plugins/cloudbees-jenkins-advisor#network-configuration for more information about insights.cloudbees.com

Also we are seeing these errors in the config page:
 

and in the admin page (monitor)
 

They could be more readable