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What are PO Box-Only Zip Codes

What is PO Box-Only Zip Codes ?

ZIP codes are postal codes developed and used by the USPS. The term ZIP stands for “Zone Improvement Plan.” A ZIP code is used to identify the post office or area where mail would be delivered after final sorting. Mail carriers use ZIP Codes to indicate delivery routes and locations serviced by the USPS. Other than being used to deliver mail, ZIP codes have many uses. Alongside tracking mail, ZIP codes have been used to help gather geographical statistics in the US. ZIP codes are also used as a security measure in credit card authorization through the Address 

Verification System (AVS)

Initially, the basic format of ZIP codes consisted of five digits. However, in 1983, an extension to the ZIP code dubbed as ZIP+4 was introduced. This new format added four digits in front of the already existing five-digit code to identify a specific area within an area covered by a 5-digit ZIP Code. 

What is a PO Box-Only ZIP Code

USPS has a classification system for ZIP codes which can be broken down into four types. The first type is the Standard ZIP code which is the most widely used type of ZIP code. The second type is a Unique ZIP code used for addresses that receive a high volume of mail. Companies, organizations, and universities usually use this ZIP code. The third type is a Military ZIP code. Military bases overseas, such as vessels or ships, use this type of ZIP code. The fourth type is a PO Box-Only ZIP code. A PO Box-Only ZIP code is assigned to specific addresses to which the USPS cannot ship mail. The USPS deems these types of ZIP codes as inadequate and, as a result, does not ship to them. However, just because specific addresses can’t receive mail does not mean they are exempted from receiving access to postal services. This is where PO Box-Only ZIP codes come in. PO Box-Only ZIP codes contain addresses that only have a PO Box address.

Several regions such as rural towns or even high-growth areas of cities where postal mail is received solely through PO Boxes. An “indicator can classify each type of ZIP code.” Regions where PO Boxes are exclusively used to receive mail, are marked and classified as “P.” USPS uses the “P” classification for every ZIP code that it does not deliver to. As a result, it is easier for USPS to differentiate and filter out addresses where physical mail delivery is impossible. 


To sum it all up, there are four main types of ZIP codes used by the USPS; Standard, Unique, Military, and PO box-Only. PO Box-Only ZIP codes are used for addresses that do not have a physical address where mail can be received. PO Box-Only ZIP codes fall under the “P” classification. This classification helps the USPS filter out addresses that it does not deliver to. The USPS keeps a database of all the ZIP codes it does not deliver to.


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Written by Austin Proctor 


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