Direct Mail

PO Box–Only ZIP Codes

By 31 March 2024April 18th, 2024No Comments
po box only zip codes

PO Box–Only ZIP Codes

ZIP codes play a crucial role in ensuring your postal mails are processed efficiently without any delays or bottlenecks. The ZIP codes enable postal services like USPS to easily identify and sort postal mails sent to a destination. Not having ZIP codes in your postal mails could cause potential delays. The postal mails that are sent without a ZIP code on them may be required to be handled manually, causing unnecessary delays that could have been avoided if you had provided the ZIP codes.

po box only zip codes

Most of us are already familiar with ZIP codes and ZIP+4 codes, and at the very least, we’ve heard about them even though you may not know how the ZIP codes work. The ZIP+4 codes give the USPS a more pinpointed location of the delivery address, allowing them to make more efficient deliveries. Sounds pretty neat, doesn’t it?

However, there is more than one type of ZIP code the USPS uses. The PO Box-Only ZIP code is one of the unconventional types of ZIP codes employed by USPS.

This article discusses what PO Box-Only ZIP codes are, what their uses are, the classification of ZIP codes, and the importance of getting your hands on accurate data.

:bulb: Also Read: Zip Code Validation

What are PO Box-Only ZIP Codes?

Sometimes mailbox mail delivery is not possible for certain addresses but they may still require access to the postal services. This is where the PO Box-Only ZIP codes come into the picture. They are used for addresses that do not have a mailbox for delivering the mails. In other words, the PO Box-Only ZIP codes contain addresses that only have a PO Box address.

There are whole areas where the postal mails are received through PO Boxes alone. In fact, each ZIP code actually comes with a “classification” indicator, and the areas that are served exclusively via PO Boxes are classified as “P.” This makes it easy for the USPS to filter out addresses that do not deliver mail to physical addresses.

One of the things that you must love about USPS is that they keep an impeccable set of records of just about everything. Hence it is no wonder that they keep a recorded list of ZIP codes that the USPS does not deliver to the physical addresses. This list of addresses are the same ones that are classified as “P,” and they are clubbed together under a special ZIP code that we call the PO Box-Only ZIP code.

Also Read: PO Box vs. Street Address

Classification of ZIP Codes

We are familiar with the regular address we use for our homes and even workplaces. From the point of view of USPS, a regular address is one that has a physical location at which their mails can be received.

For example, suppose your office is at, let’s say, “Stark Tower” and live downtown, but you often switch addresses when buying office supplies depending on your convenience.

Here the postal mail can be received at both of the physical addresses – your office as well as your home.

In this case, both the addresses are deliverable by the USPS, and they are clubbed together under the same category the USPS calls “S,” which stands for “Standard.” This is typically where all addresses that are physically deliverable by the USPS fall under.

The second category of addresses is those that are American when considered nationally but not if you consider it by the locality.

This category usually refers to military addresses. This could include ships that are stationary or military installations installed on foreign soil. This kind of address is classified as “M” by the USPS, which, as you may have guessed, stands for “Military.” These addresses are deliverable by the USPS, but their delivery system is more similar to international shipping.

Then again, there are also addresses that are truly unique in every aspect. They include companies, organizations, and institutions that regularly receive a large number of postal mails. These addresses are often given their own ZIP codes but what’s truly interesting is how they are classified by the USPS. Addresses like companies, organizations, and institutions with their ZIP code are classified as “U,” which stands for “Unique.”

The final classification of ZIP codes is reserved for PO Boxes.

The addresses that can not receive their mails directly from their doorsteps/mailbox or, more accurately, from their physical addresses receive the “P” classification. You can even call it a PO Box Every ZIP code zip code list if you may. Every ZIP code, including home and business locations that the USPS doesn’t provide their service to, is classified under this section.

How to get Accurate Data of ZIP Codes

The ZIP code data needs to be accurate, and you need a reliable source for this information. Now, if all you want to know is whether a location falls under the “P” classification, it is relatively easy to find, and businesses often prefer knowing if a particular address is a PO Box one or not. Because by the end of the day the effectiveness of your direct mail efforts comes down to where or to whom the mail was delivered. For this, they need to know whether the mail was delivered to their PO Box address or home/office address.

Luckily most advanced address verification tools such as PostGrid come equipped with the capability to find out an address’s full ZIP code. Tools like PostGrid even allow you to bulk verify addresses with a simple CSV file. Furthermore, the address autocomplete API from PostGrid can even detect the PO Box addresses in your address database and provide you with a BOX ID which makes it abundantly clear whether an address is a regular one or a PO Box one.

Also Read: Does UPS Deliver to PO Boxes


The PO Box-Only Zip codes are reserved for addresses or locations that do not have a physical address to receive their mails. Businesses often determine the effectiveness of their business operation based on the deliverability of their direct mails. Therefore, it becomes essential for businesses to determine whether an address is a regular one or a PO Box one for various reasons. Advanced address verification tools like PostGrid can verify your address database and its superior API can even tell whether an address is a PO Box address and help businesses optimize their operations.

Also Read: Can Certified Mail Be Sent to a PO Box

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