I find the message self explanatory. Outlook is server authoritative. Message does not exist on server and thus it can't be saved. But it is available on that

I find the message self explanatory. Outlook is server authoritative. Message does not exist on server and thus it can't be saved. But it is available on that client - client was able to load it before it was removed from server - and such it can be copied out or discarded from client.