The small mailboxes for the apartments fill up if you don’t empty them at least every one or two weeks. Once the mailbox is full, USPS will not deliver any more mail to your mailbox. To avoid this, you can have the post office hold your mail (officially, mail be held for a maximum of 30 days, but if you ask nicely at the Palmer Square post office they usually allow a few extra days) or request that the post office forwards your mail to another address (eg your department, but please check with them first). You can find more information about delivery options at the post office website. Please note that Princeton University has a unique zip code, so if you want to forward mail to your department address, the post office will only accept paper forms, thus you won’t be able to set up forwarding online.