Mail.app "Junk" button is disabled

A few days ago, I noticed both the "Mark as Junk" button on the toolbar and the menu item at "Message" > "Mark" > "As Junk Mail" are disabled. Furthermore, the junk mail icon indicator is no longer visible on mail that is marked as junk.

I have no idea why this is the case, but I would love to revert back to the previous behavior (functional button and visibile icon).

Any suggestions?