Postal: Mailboxes or collection boxes that are for postal use are prepped for the Postal Master Lock. This lock is only available to the Post Office and allows your mail carrier to collect or distribute mail. Multi-tenant commercial mailboxes that require a USPS Postal Master Lock have a master door that allows your mail carrier to open one main door of the mailbox and distribute the mail to all your tenants at one time. Collection boxes that use a USPS Postal Master lock have a slot for you to drop off your outgoing mail for your mail carrier to pick up later. Private: Commercial mailboxes that have a private lock can only be accessed by the owner of the box. Multi-tenant mailboxes for private distribution are most often used in offices or universities when the post office is not going to be accessing the mailboxes directly.