How Do Cluster Mailboxes Work?

Cluster mailboxes have been a standard feature in townhouse communities and apartment complexes for many years. Recently, these mailboxes have been growing in prevalence thanks to encouragement from the United States Postal Service that runs your local post office.

A cluster box unit (CBU) mailbox is a free-standing mailbox configuration that consists of multiple tenant boxes and parcel compartments that can lock individually. A cluster box can serve as commercial mailboxes or as a centralized mail delivery location for a street, neighborhood, apartment development, or subdivision.

This article looks at the different types of cluster mailboxes, how they work, and their advantages over individual roadside mailboxes.

Cluster Mailboxes vs. NDCBUs

Cluster box units and Neighborhood Delivery and Collection Box Units (NDCBUs) both provide for mail service to multiple residences. However, in the late 1990s, the post office found that the rear-loading NDCBUs had an outdated design and weren't as secure as the styles of cluster mailbox we commonly see today.

A property owner may not have been wanting to replace an existing NDCBU, but the USPS now says that these units are no longer suitable for mail service. Only CBUs are USPS-approved. In fact, the government agency says that postal service officials must not install Arrow locks on new NDCBUs, and letter carriers must not deliver mail to any replacement NDCBUs.

Different Types of Cluster Mailboxes

You will find different types of cluster boxes, but all of them allow mail delivery to multiple recipients at once.

Cluster Box Unit Mailboxes for Tenants

In this type, each tenant has a unique key for unlocking their unit's mailbox and retrieving their personal mail. Most cluster boxes feature communal parcel lockers for large packages that don't fit into individual mailboxes. If the tenant notices a key to the parcel locker in their mailbox, they can use it to open the parcel locker and access their package.

After unlocking the parcel locker, the key remains in the lock. The postal carrier will then remove the parcel locker key during the next delivery.

Check out the video below for more information:

Cluster Box Mailboxes for Postal Carriers

The design of cluster boxes allows for quick and convenient mail delivery. The postal carrier uses a proprietary Arrow lock to open the unit's entire front panel, providing access to every individual mailbox. The USPS is responsible for installing and maintaining the Arrow locks.

The postal carrier can then insert the mail into each slot without having to open individual mailbox doors. After delivering all the mail, the postal carrier can close and lock the front panel securely.

The Complete Breakdown of CBU Mailboxes 

CBU Configuration

When installing a new cluster mailbox, developers can choose CBU configurations consisting of eight, twelve, thirteen, or sixteen compartments. Each compartment has a mail slot through which tenants can access their mail. Besides individual mailboxes, a CBU also features one or more parcel lockers for the delivery of large packages.

Each CBU has a front panel or master door that the postal carrier can unlock with a master key. Opening this panel provides access to all the compartments in the cluster unit.

CBU Keys

A cluster mailbox typically has three keys per box. In most cases, each tenant will receive two keys, and the property manager will receive one. The post office has the master key that unlocks the front panel, and they will issue this key to the postal carrier for the delivery of mail.


If you are not familiar with mailbox clusters, you may be worried about your mail and parcels' safety. Fortunately, cluster mailboxes are generally safer than roadside ones as they can lock so that others cannot access your box through the mail slot.

Another common concern is sharing parcel lockers with other tenants. However, if you have a package in the parcel locker, you are the only one who has the parcel locker key, and no one but you can open the parcel locker door.

Security features of cluster boxes include reinforced corners and aluminum wrapping to prevent intruders from prying boxes open. Each mail slot also has an anti-fishing mesh and hood to deter mail theft.


Most cluster mailboxes stand outside in the sun and rain, and a protective finishing is critical to prevent corrosion. The CBUs from Budget Mailboxes have a component primer layer with a powder coat finish to protect the unit from the sun's UV rays, water, and outdoor temperature fluctuations. The finish contributes to the cluster mailbox's functional lifespan, so you don't have to replace the mailbox.

You have a choice of several available finishing color options, including green, white, black, bronze, grey, and sandstone (by far our most popular color). Property developers who prefer an authentic mailbox appearance can opt for the finial cap and traditional pedestal instead of the pillar pedestal. 

CBU Regulations

All CBU mailbox sites and equipment should be USPS-approved. The U.S. Postal Service National Delivery Planning Standards document includes these regulations for builders and developers. All the USPS-Approved Cluster Mailboxes for sale on Budget Mailboxes will showcase the USPS seal of approval.

Appendix A and B include installation guides to ensure proper foundation preparation and mounting procedures. The standards document also lists location requirements to ensure optimal tenant convenience and safety.

Are Cluster Mailboxes Safe?

Whether street-side mailboxes, boxes located centrally at an apartment complex, or ones housed within office buildings, cluster box units have several USPS-approved security features. These guard against identity theft and stolen shipping packages.

For starters, only mail carriers have the proprietary keys that unlock and open the entire front panel on CBUs, which provides access to every mail compartment for each resident or user. Residents receive individual compartment keys to open their own mailbox door, not the doors to other mail boxes.

A cluster mailbox also typically features a package compartment for things that are too large for any mail receptacle. The mail carrier places a package box inside the locker and then drops the key for the locker inside the recipient's box. The recipient can use that to open the package compartment, but a USPS-approved lock will then trap the key until the carrier can retrieve it.

How Do I Send a Letter from a Cluster Mailbox?

All cluster mailboxes have an outgoing compartment for posting outgoing mail. The postal carrier will retrieve outgoing mail with each delivery.

How Do I Get a New Cluster Mailbox Key?

After installation, each tenant should receive a key to their mailbox as well as a spare. The property manager must also have copies of all the individual mailbox keys. If you recently moved in, the previous tenant or property manager should give you the keys to your mailbox.

If you accidentally lose the keys to your mailbox, ask the property manager for a replacement. The manager will give you their copy to make a duplicate, or they will take the key to a locksmith to make duplicates. If the manager doesn't have a key to your mailbox, an entire lock replacement may be necessary.

How Do I Determine Which Mailbox is Mine?

All individual mailboxes should have an adhesive placard or engraved number or figure as identification. Property management can use any identification system so that tenants will know which mailbox belongs to them.

Why Does USPS Care So Much About Mailboxes?

Delivering mail to a central location benefits both customers and the USPS. A cluster mailbox gives mail carriers one stop instead of having to search along their route, stopping at box after box. This saves letter carriers time, fuel, and wear and tear on their vehicles.

In addition, the USPS prides itself on being dependable and reliable. It doesn't want packages to disappear during shipping any more than a customer does. Because of their manufacturer-installed safety features, CBUs also have more security than the traditional roadside or doorstep mailbox.

Who Is Responsible for Cluster Mailboxes?

USPS regulations say that property owners, developers, or builders are responsible for installing, maintaining, and repairing cluster mailboxes. However, many property owners appoint an employee to check that the cluster box unit meets USPS guidelines.

Other FAQs

Are Cluster Mailboxes Required?

The USPS requires central mailboxes for all new or remodeled apartment houses. This requirement has been in place since 2019, in spite of objections from groups such as the National Association of Home Builders.

The USPS sets requirements for installation of the box, although property owners can choose from among several designs. However, the USPS says that for every five mailbox compartments, at least 1 parcel locker is necessary.

How to Open a Cluster Mailbox Without a Key

Cluster mailboxes have a design to enable each customer to receive two keys to their box: an everyday one and a spare. But sometimes, a customer moves and forgets to return these. Other times, a customer might lose one or the other.

These mailboxes have features to prevent tampering, such as creating an opening in an individual box by prying around the edges. If you no longer have mailbox keys, ask the property manager about a replacement. They may make duplicates using a locksmith for a fee. If the keys are gone because a customer moves, the manager might want to replace the entire lock for additional security.

Ordering a Cluster Mailbox from Budget Mailboxes

If you want to order a cluster mailbox, Budget Mailboxes can help. We will help you select the unit features you need for your subdivision, community, business, or apartment complex. We will also provide you with all the information you need to ensure the correct installation.

Contact us today at 866-707-0008 to place your Budget Mailboxes order. Bulk discounts are available!