A private mailbox, or mail center mailbox, is typically inside a building, behind a door that is locked from the outside. Mailboxes are only accessible to named users who have a key. Secure 24/7 lobby access means that mail can be picked up at any time. In many cases, a mail center is more secure than using your residential mailbox. Most residential mail is delivered to an unlocked mailbox outside a home. Mail delivered to a mailbox in a mail center has two layers of security: the building entrance and the mailbox itself.