Try removing and re-entering the address you're sending to. Sometimes that works. I also found this information:
Advanced logging for the SEPM console can be enabled by:
1. Stop the Symantec Endpoint Protection Manager service
2. Add the line scm.log.loglevel=FINEST and scm.mail.troubleshoot=1 to the bottom of the file:
C:\Program Files\Symantec\Symantec Endpoint Protection Manager\tomcat\etc\conf.properties
3. Restart the Symantec Endpoint Protection Manager service
Once logging is enabled, search this log for the email address the notification should go to:
C:\Program Files\Symantec\Symantec Endpoint Protection Manager\tomcat\logs\SecurityNotifyTask.log
-or-
C:\Program Files\Symantec\Symantec Endpoint Protection Manager\tomcat\logs\catalina.out
If you don't see any errors in this, then you may want to check the mail server logs.
sandra