The phrase "new mailbox" refers to a brand new box where you can receive letters and packages. It implies that you have recently gotten a mailbox or replaced your old one with a new one. Full definition
Improving your company's profile by installing new mailboxes, building new post offices and delivering on time, every time (thatvideogameblog.com)
Created user accounts in Active Directory for new employees along with new mailbox creation in Microsoft Exchange management Console (jobhero.com)
But in this game, players can also adjust the specific look of thousands of individual pieces of furniture, including new types of wall items like clocks, shelves and lamps, and decorate the exterior of their houses with new mailboxes, doors, façades and stepping stones. (wiiloveit.com)