Every email list manager faces the challenge of handling invalid email addresses or bounce messages from the list. Most of the time, the bounce message that you receive indicates that the email address no longer exists or that the server being sent to was unavailable. Usually, the best first course of action is to verify that the email address referenced in the bounce message is correct.
For more information on the particular bounce messages and how to respond are found in this document from L-Soft: bounce handling.
For information on monitoring alerts, see the related article in the link to the right.