What you should know about locking mailboxes, and how they work
Many people ask us if their postal carrier will need a key if they buy a locking mailbox. The answer is no. In fact, letter carriers are not allowed to carry a key for residential mailboxes.
Most USPS approved residential curbside locking mailboxes work in the same basic way as an unlocked curbside mailbox. The postal officer deposits your mail through an incoming mail door or slot. This door is not locked, and is usually big enough to accept all your mail and small parcels, but should not be large enough to allow a fishing hand to reach in. (See for example this article).
Once your mail is in a secure locking mailbox, the incoming mail is protected from would-be mail and identity thieves. For true mail security, the incoming mail slot should be too small for prying hands to reach in and fish out your mail. For larger, commercial size boxes, the mailbox must be designed to prevent reaching through the incoming mail slot. Additionally, a secure locking mailbox should feature an anti-pry mechanism such as the Mail Boss anti-pry latch to prevent leveraged entry. Otherwise, the mailbox can be easily pried open in seconds with a screwdriver or other household items.
With a locking mailbox, the homeowner removes their mail with a key by unlocking the mail-removal door and retrieving their mail. Depending on the model and style of the mailbox, the key-locked door may be in the front, rear, or both.
While a well-built locking mailbox like the MailBoss security locking mailbox can provide security for your incoming mail, no USPS approved locking mailbox protects your outgoing mail. Generally, outgoing mail is handled by placing it in a separate compartment near the incoming mail door. The mail may be held by a clip or sit inside the door (credit jerome). Since the outgoing mail must be available to the letter carrier, the door is not locked.
Red flags are used to signal to the postman that there is outgoing mail, but these red flags also signal to mail thieves that there is a wealth of material ripe for the plucking. Thieves target “flagged” mailboxes because they often include bill payments with account information and checks which they can wash and reuse fraudulently. For this reason, we recommend depositing your outgoing mail in a USPS blue box or other secure location.