My hunter has a persistent mailbox flag, but talking to the postmaster gives the "no messages" screen. The flag appears both in The Tower and at The Reef, and survives delivery and removal of other items through the mailbox. It's about a week old, now, which means it has, I think, survived one server reset. It appeared after I donated a bunch of motes to level up my New Monarchy status; my first thought was that a shader or something had overflowed from my full inventory into the mailbox, but it was just a flag with nothing there. And I'm still seeing that flag every time I walk past a postmaster, though it does not appear from Director view.