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Everything You Need to Know About USPS Cluster Mailbox Regulations

In a way, a mailbox is an unsung hero in our day-to-day lives.

Think about it. It brings us joy on our birthday and anxiety when the bills come in. It helps keep our country running. For all the technology we have in our world, nothing beats getting a package or good news in the mail.

Even if you don’t think about the importance of mailboxes, the USPS sure does! In fact, they have several guidelines regarding the size, placement, and maintenance of mailboxes.

If you’re a residential or commercial property manager, your cluster mailbox has even more guidelines to follow. In this article, we’ll tell you everything you need to know about USPS cluster mailbox regulations.

Why Does USPS Care so Much About Mailboxes?

The history of mailboxes is both fascinating and informative. While there was postal service under the British Parliamentary Post, U.S. Mail first starting operating as an independent, separate from England in 1775.

As strange as it might sound, the first mailboxes weren’t used until the 1850’s. But they aren’t the mailboxes we know today.

The first mailboxes date back to 1833 and were only used for collecting mail. U.S. Mail letter carriers started dropping boxes off along their routes so people didn’t have to go to the post office to mail their letters. The convenience cost two cents per letter and only lasted a few years.

But those who lived in rural areas, miles away from post offices set up in town centers, weren’t too pleased the service stopped.

As the United States’ population continued to flourish, hand-delivery became cumbersome for carriers. Some businesses and residences set out tin cans for the carrier to pick up and deliver their mail into.

But nothing got regulated or was widespread until 1858. A Philadelphia merchant petitioned the city to allow his cast-iron letterboxes to hang from lampposts and the city agreed. Not too long after, Boston and New York followed suit.

It wasn’t until 1923 that it became mandatory for every household to have a mailbox or a door slot for the mail. The USPS added size restrictions and later, placement guidelines got added.

It may seem like nit-picking, but a mail carrier has a lot of mail to deliver to a lot of households and businesses. If there weren’t regulations stating the placement or height guidelines, it could take a carrier a long time to deliver his or her entire route.

So, part of the reason is the convenience, but speed and efficiency on the part of the USPS play a role too.

What is a Cluster Mailbox?

A cluster mailbox, or cluster box unit (CBU), is a multi-unit mailbox centralized for communal use. They consist of at least eight locked compartments that get mounted onto a pedestal. They are customizable to meet specific needs and to blend into surrounding building aesthetics.

Office buildings and multi-family residences use cluster mailboxes for convenience and security purposes. You’ve seen them in complexes big and small and even newer neighborhoods have CBUs.

In general, all cluster mailboxes work the same way. The carrier has a master key that opens the back of the unit or all the compartments at once. After placing the mail in the proper compartment, the carrier locks the CBU up and individual box users access their particular box with a key.

Safety is a growing concern among citizens as CBUs pop up across the country. But whereas a curbside or porch mailbox doesn’t lock, leaving your mail out in the open, mail delivered to a CBU is safe from tampering and the elements.

What are the USPS Cluster Mailbox Regulations?

The USPS has set several guidelines in place for cluster boxes but we’ll discuss the most common questions relating to CBUs.


If a private party owns the CBU and it’s used by a household, condominium, apartment complex, or other type of residence, customers get their key from the owner, manager, or prior resident. The builder or property owner provides lock and key service for cluster mailboxes owned by private entities.

If the USPS owns the cluster box, the Postal Service provides every customer a compartment and three keys for the individual unit free of charge. The USPS does not keep copies of the keys and not does charge a deposit for use of the CBU or keys. If the keys get lost, USPS installs a new lock and keys at the customer’s expense.

If the customer moves, they must return all three compartment keys to the post office that handles that address. The USPS will change the lock and keys to the compartment before reissuing it to a new resident.

Mail Service to a CBU

Postal carriers must deliver mail to an address as it’s written. Sometimes, this means you receive a prior resident’s mail. If this happens, don’t tamper with the mail and instead, there are several ways to handle the situation.

  • Leave a note inside your compartment with a note saying, “[Name] doesn’t live at this address” or “Only [your name] lives at this address.” The carrier should make a note of the change and handle it moving forward.
  • Write “Return to sender” or “Not at this address” on the envelope and put it in the outgoing mail slot.
  • Visit your local post office and request that you stop receiving the prior resident’s mail at your address.

If you live in a multi-family unit, this may happen often especially when you first move in. Note that it’s illegal to stop someone else from getting their mail, so at the very least, write a quick “RTS” on the unopened envelope.

Mailbox numbers don’t have to correspond to the actual address and, in most cases, have the resident’s name marked inside. In other words, if your address is 123 Main Street, your unit could have “123” marked on it or it could have another numerical designation.

In most cases, cluster box units have parcel lockers as well as smaller compartments for letter mail. Parcel lockers are for packages and larger envelopes that don’t fit inside the regular compartment. The carrier will leave a parcel locker key inside the letter compartment.

A number or letter is on the key tag that coincides with the parcel locker a package gets delivered to. The resident or customer uses this key to get their package. Once a resident retrieves the package, the key stays inside the lock and close the parcel locker door.

Most CBUs include an outgoing mail slot. This is where you drop your mail and the carrier will collect it during his or her route and take it for processing. This is at no cost to you.

We’re Leading the Way

Now that you know all the basic USPS cluster mailbox regulations, you can start your buying process! If you’re not sure where to start, we can help.

At National Mailboxes, a division of NMHP Inc., we’re a leader in the commercial mailbox industry. We’ve manufactured and supplied high-quality cluster mailboxes all over the country for decades.

Browse our product line or contact us today for a quote.

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