Notification sent from IM contains an unknown recipient, that is not on a list of users that should be notified.
It may happen with approval notification for example
Release : 14.X
Component : Identity Manager, Identity Suite
One of the proper notification recipients has a Delegator/Delegate set up via Out of Office Assistant.
Check that an unknown recipient is a Delegator.
See documentation for your version of IM:
https://techdocs.broadcom.com/us/en/symantec-security-software/identity-security/identity-manager/14-4/administrating/workflow/work-lists-and-work-items/delegating-work-items.html