Release : 15.7 MP2+ , Mac OS 10.15.x, Mac OS 11.x
Component : Outlook 16.30+
This is a known limitation of the on-send add-in type for Outlook.
This is a known limitation (external link) of the Microsoft OnSend add-in type for Outlook on Mac OS, and is not specific to the Symantec DLP add-in
Currently there is no workaround to this other than to disable Offline Mode as an option for Outlook Users on Mac OS.