How can I update mail to show a "warning" message that exists on a users' mailbox as like what you see when mail is moved to the "junk mail" folder using the Graph API? See attached screenshot. Obviously, I want this message to be customised with my own text. It's not possible to update messages if they aren't draft (isDraft=false). I dont think that Transport rules are good for us because I want to add that label after the message has arrived into the users' mailbox. I know that its possible to add Custom Attributes to a message resource - https://learn.microsoft.com/en-us/graph/api/resources/opentypeextension?view=graph-rest-1.0
Can I utilize it? any way that you can recommend to do such a thing? Do I need to develop any addon for that?
After speaking to the PM , there is no supported scenario to do this. If you require this feature you can make a request on https://microsoftgraph.uservoice.com/