A Response Rule that is configured with a Send Email Notification to notify the sender their message has been blocked fails to reach the sender.
15.8.x, 16.0.x
If the first attempt by the DLP Enforce server to send the message fails because the SMTP server is too busy to accept it, then the message will not be retried by the Enforce server and therefore is missed.
This is a product limitation, in the current DLP design there are no options to specify and configure a number of retries or a specified retry interval for resending the notification email again if at first it fails.
We have an open feature request to add such a feature in a future release, which is under planning. If you would like to be an endorser for this request, please contact Technical Support and request your organisation be added to the feature request ISFR-2898 - DLP - Retry interval for failed email notifications sent via response rules.