My application displays a folder structure in a tree. The user can browse the contents in these folders, and drag content into the folders.

However, some of these folders are readonly (meaning no content can be dragged into them). I'm looking for a clear way of depicting this to the user, so they are aware which folders they can drag to before actually having to try it, i.e. hover state. Something not too alarming or interrupting (no big red crosses overlayed on top of the folder icons, for instance), preferably a recognized standard.

Specifically, we are talking about an Outlook add-in showing a SharePoint folder tree here, but I'd much rather get the answer from a generic viewpoint. From the top of my head, I simply don't seem to know any applications that visually mark readonly folders.