Mailboxes have evolved over the years. They are no longer limited to traditional curbside designs. While curbside mailboxes are still used, many residences and businesses are now being developed with STD-4C mailboxes. The successor to the United States Postal Service's (USPS's) STD-4B standard, it's a centralized mail delivery system that includes multiple mailbox units. If you're thinking about buying STD-4C mailboxes, though, there are a few things you should about them.