An Outlook user may report the following error when they try to modify their Delegates settings.
The Delegates settings were not saved correctly. Cannot activate send-on-behalf-of list. You do not have sufficient privileges to perform this operation on this object.
This error can occur when the Delegates list contains a mailbox user who no longer exists in the organization. To fix the error remove the non-existent user from the Delegates list before you attempt to add other Delegates or change the Delegates settings.