Mail system broken? No bills, no letters, no mail carrier

I've been having an issue for a couple of weeks now where my households don't receive any bills or mail. I've tried repairing the game, starting new save files with completely new households, removing and replacing the mailbox, and even using testingcheats to force both mail and the mail carrier—but nothing shows up. I've also tried removing all mods and custom content, but that didn't solve the problem either.

Does anyone know what might be causing this? What can I do to fix it? It's really driving me crazy at this point!