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Salsbury Mailbox Blog

What is a Centralized Cluster Mailbox?

April 14, 2023

If you are in the market for new mailboxes for your community or business, you have likely heard of centralized cluster mailboxes. Centralized mail is the form of delivery preferred by the United States Postal Service, after all. Cluster box unit specifications are determined by the US Postal Service and became the standard for every manufacturer.

But you still may be unsure of what exactly a centralized cluster mailbox is. Clustered mailboxes consist of multiple mailbox compartments and parcel lockers that can lock individually. They include free-standing pedestal-mounted Cluster Box Units (CBUs) or other cluster mailboxes mounted in a shelter, kiosk, or wall. Centralized delivery simply refers to a mail delivery system in which the mail carrier delivers mail for multiple residences at one central location.

So, a centralized cluster mailbox is one with multiple compartments and lockers where tenants or employees can access their mail in one convenient, central location. It makes sense that the USPS would prefer this form of delivery since it cuts down on costs, increases security, is better for the environment, and more (as we’ll discuss below).

Now that you know what a centralized cluster mailbox is, there is still plenty more to learn before getting one for your community or business. To help you through the complicated and sometimes frustrating journey of purchasing a centralized cluster mailbox, here’s a breakdown of the different types of centralized cluster mailboxes, USPS requirements, benefits, and how much they cost.

Types of Centralized Cluster Mailboxes

There are multiple different types of centralized cluster mailboxes available. The most commonly used are CBUs and 4C mailboxes, though 4B can also be used in some cases.


The USPS defines a CBU as a “freestanding, pedestal-mounted mailbox containing 8, 12, 13, or 16 individually locked mailboxes and parcel compartments”. Each Salsbury CBU includes 4, 8, 12, 13, or 16 mailbox compartments with one, two (2), or four (4) fully integrated parcel lockers. Cluster Box Units are easy to install and will make an attractive addition to your community.

These types of mailboxes are generally placed outside, so that’s something to keep in mind when making your purchasing decisions. They are best suited to neighborhoods, complexes, or subdivisions.

4C Mailboxes

USPS STD-4C horizontal mailboxes are centralized, wall-mounted mailboxes that are required for new construction and major renovations. A “major renovation” refers to when the wall where the existing mailboxes are installed is being substantially modified. Salsbury Industries manufactures recessed mounted 4C horizontal mailboxes and parcel lockers, and offers them in standard and custom units.

Unlike CBUs, 4C mailboxes are typically placed indoors. They are ideal for apartment and office buildings.

4B Mailboxes

4B horizontal and vertical mailboxes are specifically for direct one-to-one replacements. 4B horizontal and vertical mailboxes can be used for replacement use if no changes are being made to the rough opening where the existing wall-mounted mailboxes are currently installed.

You should always check with your local post office to be sure that you can use a 4B mailbox. Otherwise, a 4C mailbox must be used. Like 4C mailboxes, 4B mailboxes are usually placed indoors.


USPS Cluster Mailbox Requirements

There are several benefits that make centralized cluster mailboxes a worthwhile addition to your community. Here are just a few of them.

They Are Better for the USPS

USPS mail carriers have to deliver to a large number of homes, and that number is growing all the time. It is much easier and more efficient for mail carriers to deliver the mail to one centralized location than driving from house to house. This type of mailbox also cuts costs for the USPS on salaries since each mail carrier will be able to cover more ground.

Better for the Environment

Avoiding driving from house to house doesn’t just make things more efficient for your mail carrier. It cuts down on fuel and results in greener neighborhoods.


Salsbury centralized cluster mailboxes are made of heavy-duty aluminum and stainless steel hardware. With their corrosion resistance, you won’t have to worry about spending time and money on constant maintenance. These mailboxes will retain their good looks for years to come.


Larger Compartments

Centralized cluster mailboxes have bigger compartments, so residents won’t have to worry about damage to larger envelopes and small packages. They also won’t have to trek out to the post office to collect small packages that won’t fit in their curbside mailboxes.

Increased Security

Centralized cluster mailboxes are more visible and less isolated, making theft less likely. They also have several features that will make residents feel safe. In addition to being constructed of strong, sturdy materials, each mail compartment’s door comes with a heavy-duty cam lock and includes three keys. There is an anti-fish plate as well on the rear of the mail slot that will prevent tampering with outgoing mail.

Parcel lockers also save residents concern over their packages being stolen. When a mail carrier delivers a package that doesn’t fit in a resident’s mail compartment, they put it in the parcel locker and place the parcel key in the recipient’s compartment. After the recipient takes their parcel, the key capture system locks the parcel key in place. Then the parcel key can only be removed with the USPS control key.


These mailboxes will make a far better-looking addition to your community than curbside mailboxes. Check out our Regency Decorative Cluster Mailboxes to add a particularly classy touch to your neighborhood.

As you can see, there are a ton of benefits to adding a centralized cluster mailbox to your community or business.


How Much Does a Cluster Mailbox Cost?

One question you’ll inevitably ask yourself during the purchasing process is how much a centralized cluster mailbox costs. The answer is that the cost can vary quite a bit depending on the type of mailbox you’re looking for and the number of compartments.

Now that you have a better idea of what centralized cluster mailboxes are, the different types, the USPS requirements and the benefits, you should be able to confidently go forward with purchasing CBU4C, or 4B mailboxes for your neighborhood or business.

If you’re still not sure which mailbox would suit your needs, you can try out our Mailbox Configurator, which will help you determine the optimal number of cluster mailboxes or 4C Horizonal Mailboxes needed based on your specific requirements. You can also contact us to request a quote today to help you figure out the unique centralized cluster mailbox that will work best for your community.