Navigating the complexities of mail delivery can be challenging, especially when dealing with the United States Postal Service’s (USPS) guidelines and systems.
Whether you’re a small business owner, a marketer, or simply someone trying to send a letter, understanding how the USPS uses the Coding Accuracy Support System (CASS) to certify address lists and the reasons behind address rejection or non-delivery is crucial.
This article explores the common causes for address rejection under the CASS certification USPS uses and examines the implications of vacant homes and mail forwarding on mail delivery.
Why Addresses Get Rejected: The CASS Certification USPS Process
CASS certification is a system the USPS uses to evaluate the accuracy of address-matching software.
This system plays a vital role in ensuring that addresses are correctly formatted and standardized according to USPS guidelines. However, there are several reasons why an address might be rejected during this process. Let’s take a look at these reasons.
Incorrect Address Elements
Missing or incorrect components like street names, numbers, cities, or ZIP codes.
Invalid or Nonexistent Addresses
Addresses that do not exist or are not recognized in the USPS database.
Formatting Errors
Non-compliance with USPS-specific formatting requirements.
Lack of Standardization
Failure to use standardized formats to address components.
Mismatched ZIP Codes
Discrepancies between ZIP codes and the associated city or state.
Obsolete Addresses
Changes in street names or renumbering that are not updated in the system.
Lack of +4 ZIP Code Extension
Missing extensions in addresses where they are necessary.
Rural or Non-Standard Addresses
Challenges in standardizing addresses in less developed or rural areas.
P.O. Box Issues
Incorrect or missing P.O. Box numbers.
Apartment/Suite/Unit Number Problems
Missing or incorrect details in multi-unit building addresses.
Vacant Addresses and Forwarded Mail
Apart from the CASS certification hurdles, there are other scenarios where mail delivery might be affected.
Vacant Addresses
If a property is marked as ‘vacant,’ USPS may suspend mail delivery because there’s no one to receive the mail. This is common in properties that are between tenants or owners.
Forwarded Mail
When residents set up mail forwarding, mail is sent to a different address. In this case, mail not part of the forwarding request might not be delivered to the original address.

Best Practices for Ensuring Mail Delivery
Regular updates to address lists using services like the National Change of Address (NCOA) database are essential to minimize these delivery issues. This database tracks address changes and forwarding requests, helping to ensure mail reaches its intended destination.
For blanket marketing campaigns, services like ‘Occupant’ or ‘Resident’ mailing can be effective, as they send mail to an address regardless of the specific resident name or vacancy status.
Navigating CASS Certification USPS Mail Delivery Issues
Understanding the CASS certification USPS address verification system and being aware of issues such as vacant addresses and mail forwarding are essential for successful mail delivery.
By staying informed and regularly updating mailing lists, you can significantly reduce the risk of mail delivery issues and ensure that your communications reach their intended recipients efficiently and effectively.
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Choose from Wise Pelican’s postcard template selection or supply your design. Effortlessly monitor your postcard campaigns and track delivery times. We use the USPS NCOA Link database to update addresses for every mailing. You can be confident that your postcards will be delivered reliably, alleviating any concerns about navigating the USPS guidelines and systems.