Updating destination list delivery server has failed
To review the value of the performance counter that generated this alert, in Operations Manager, double-click this alert, and then click the General tab.
Review the description of the alert that includes the variables specific to your environment.
These tools can help make sure that your configuration aligns with Microsoft best practices.
They can also help you identify and resolve performance issues and improve mail flow.
We assume that an email will be delivered to a recipient if no "undeliverable" message is returned to you.
However, this is not a safe assumption as large ISPs do filter email messages without returning "undeliverable" message (email blackhole).
This can also happen when some firewall or other 'bloatware' software on the delivery machine is enabled or changed and now it's blocking the communication for Scanrouter lite.