Periodically we'll receive the below output when a job should send an email using the email-ext plugin. Since this only happens sometimes, we're not sure what the cause is.

Email was triggered for: Always
Sending email for trigger: Always
Sending email to: <LIST REMOVED FOR SECURITY>
Error sending to the following VALID addresses: <LIST REMOVED FOR SECURITY>

The problem is that since we are in production, we cannot enable debug mode since this plugin has help that says specifically that it "should not be used in full production mode."

This enhancement is to add more helpful output to give the reader of the log at least some level of confidence as to what might have caused the problem. Was the issue network related and if so what specifically(mail server unavailable, network unavailable, connection timed out, connection reset, authentication failed, server rejected, etc), or is it something else entirely?

Source location of error message:
https://github.com/jenkinsci/email-ext-plugin/blob/master/src/main/java/hudson/plugins/emailext/ExtendedEmailPublisher.java#L359