Starting tomorrow morning, you should start receiving notifications when your patrons have undeliverable email notices.  These messages will be delivered to the aliased addresses (<3 letter code>@notices.swanlibraries.net).  Please review the documentation at https://support.swanlibraries.net/system/files/Members%20Only/Circulation/Bounced%20email%20notification%20and%20tracking%2020180521.pdf for information on what you should expect and actions that you can take with the data provided.  Please note that we are unable to determine what the contents of the missed notification was at this time.  If you have any further questions, please open a support ticket.