What is CASS certification? Coding accuracy support system

Bad address data is expensive. A missing apartment number, outdated street name, or incorrect ZIP Code can turn a carefully planned mailpiece into wasted print and postage.

Coding Accuracy Support System (CASS™) certification gives direct mail teams a way to evaluate the software handling their address data. The voluntary United States Postal Service program tests whether address-matching software meets USPS standards for correcting, standardizing, and coding addresses.

Here’s what CASS certification covers, how the process works, and what to look for when choosing a direct mail provider.

What CASS certification means

CASS certification applies to address-matching software, not to a company as a whole. When software is CASS certified, it has passed USPS testing for its ability to accurately apply postal data such as five-digit ZIP Codes, ZIP+4® Codes, delivery point codes, and carrier route codes.

You’ll also hear people refer to a “CASS-certified mailing list.” USPS does not certify each list. The phrase means the addresses on the list were matched and coded using current CASS-certified software.

That distinction matters when you’re evaluating a provider. A provider should be able to confirm which product is certified, which CASS cycle it passed, and when the certification expires.

How the USPS CASS certification process works

The USPS CASS certification process has two stages.

• Stage I is an optional self-test. Developers process a file of approximately 150,000 test addresses and compare their results with the answers supplied by USPS. This stage helps them find and fix problems before the graded test.
• Stage II is required. Developers process another test file of approximately 150,000 addresses, but USPS does not provide the answers. The completed file goes back to USPS for evaluation.

To pass Stage II, software must meet USPS accuracy thresholds for the postal codes and address-quality functions included in its certification. USPS currently requires at least 98.5% accuracy for ZIP+4, five-digit ZIP Code, carrier route, and LACSLink® results, along with 100% accuracy for functions such as delivery point coding, DPV®, eLOT®, and RDI™ when applicable.

Certification lasts through the current USPS certification period. The current CASS Cycle O expires July 31, 2028. Developers must reapply and pass Stage II testing to remain certified for a new period.

What CASS-certified software checks

CASS-certified software improves the accuracy and completeness of address data before mail enters production. Depending on the product’s certification and capabilities, it can standardize address elements, correct certain errors, append missing postal codes, and add data that supports USPS sorting.

Address standardization and matching

Address standardization puts address data into USPS-preferred formats. For example, software might correct a misspelled street name, use the appropriate street suffix, standardize a directional, and fix an incorrect city or ZIP Code.

Matching then compares the standardized record with USPS address data. This process helps determine whether the address can be assigned the correct postal codes.

Standardization alone does not guarantee deliverability. An address can be formatted correctly but still point to a location that does not receive mail. That’s why CASS workflows also use address-quality tools such as DPV.

ZIP+4 and delivery point codes

The first five digits of a ZIP Code identify a general delivery area. The additional four digits narrow the destination to a more specific segment, such as one side of a street, a group of apartments, or a high-volume recipient.

CASS-certified software can append the correct ZIP+4 Code and delivery point information. USPS sorting equipment uses these codes to route mail more efficiently and reduce manual handling.

DPV

Delivery Point Validation checks whether a ZIP+4-coded address appears in the USPS delivery file as a known delivery point. This is the step that helps answer the practical question: Can USPS deliver mail to this specific location?

DPV can also flag address problems that need attention, including missing or invalid secondary information. A valid building address, for example, may still need an apartment or suite number before a mailpiece can reach the intended recipient.

LACSLink and SuiteLink

LACSLink helps update addresses that have changed because of local address conversions. This commonly includes rural-style addresses replaced by city-style street addresses, as well as streets that have been renamed or renumbered.

SuiteLink® improves business addresses by adding known secondary information to high-rise addresses. That extra detail helps mail reach the correct company or office within a shared building.

eLOT and RDI

Enhanced Line of Travel, or eLOT, assigns sequence information based on the order in which a carrier serves addresses along a route. Mailers can use this information to prepare eligible mail more efficiently.

Residential Delivery Indicator, or RDI, identifies whether USPS classifies an address as residential or commercial. That distinction can support data enrichment and affect planning or pricing for some shipments and services.

How CASS-certified software helps your direct mail program

CASS processing does more than clean up formatting. It protects your budget and makes address quality part of the production workflow.

Reduce wasted print and postage

Every undeliverable mailpiece carries costs. You pay to design, print, and send a piece that never reaches its recipient. Correcting and suppressing bad addresses before production helps prevent those costs instead of discovering the problem through returned mail.

Strong address data quality checks also protect campaign reporting. When fewer pieces fail because of bad data, delivery and response metrics give you a clearer view of actual campaign performance.

Support eligibility for USPS automation prices

Certain discounted USPS prices require mail to meet automation standards. Those standards can include using addresses processed with current CASS-certified software, applying the correct barcodes, and meeting mailpiece design and volume requirements.

CASS processing does not automatically qualify every mailing for a discount. It supports eligibility by providing accurate address and delivery point codes. Your mail class, preparation, presort, volume, and other requirements still matter.

Catch errors before they spread

If you wait until a campaign is ready to print before checking addresses, bad data has already moved through your systems. Real-time verification catches problems when customers submit addresses through a form, while batch verification cleans existing lists before a send.

An address verification API can connect those checks to your CRM, ecommerce platform, or campaign workflow. Your team spends less time fixing records manually, and fewer errors reach production.

CASS certification vs. NCOA processing

CASS and National Change of Address, or NCOA, processing solve different address problems.

CASS-certified software standardizes an address and matches it with USPS postal data. NCOA processing compares names and addresses with USPS change-of-address data to identify people, families, or businesses that have moved.

An address can pass CASS processing and still belong to someone who no longer lives there. CASS certification does not replace NCOA processing or another USPS-approved Move Update method. Direct mail teams often need both to keep addresses accurate and current.

How often should you process a list with CASS-certified software?

For USPS automation prices, an address list generally must be matched and coded with CASS-certified software within 180 days before the mailing date. If you reuse a list after that window, process it again with current software before relying on it for automation eligibility.

Treat 180 days as a maximum window, not a data-hygiene schedule. Addresses can become outdated at any time. Verify records when they enter your system and process lists again close to each campaign, especially when the data comes from multiple sources or has not been used recently.

Is CASS certification enough when choosing a provider?

No. CASS certification proves that an address-matching product met USPS accuracy standards during testing. It does not tell you whether the provider can support the rest of your direct mail operation.

Ask for current proof of certification, then look at the capabilities around it.

Real-time and batch address verification

You need different checks at different points in the workflow. Real-time verification helps prevent bad data from entering your systems. Batch verification lets you process an existing database or campaign list before production.

Look at the results the provider returns, too. A simple valid-or-invalid response gives your team less control than detailed deliverability data that identifies missing units, incorrect secondary information, vacancies, or other specific issues.

Direct mail production on the same platform

Separating address verification from campaign production creates another handoff for your team to manage. When both functions run on the same platform, you can verify recipients before printing without exporting lists or coordinating between vendors.

That connection also makes it easier to apply suppression rules consistently and keep bad addresses out of production.

Integrations and automation

Address quality works best when it runs inside your existing workflow. Look for APIs and integrations that connect with the systems where address data originates, including forms, CRMs, ecommerce platforms, and customer databases.

The right setup lets your team verify addresses automatically instead of relying on occasional manual cleanups.

Personalization and templates

Clean address data gets a mailpiece to the right mailbox. Personalization helps make the message relevant once it arrives.

Choose a platform that can use customer data to customize copy, imagery, offers, and calls to action at scale. A flexible direct mail template gallery can help your team move faster without limiting campaign design.

Production and delivery visibility

CASS certification does not show what happens after an address is verified. Your provider should also give you visibility into production, mail acceptance, and delivery events so you can identify delays and measure campaign execution.

Go beyond CASS certification with Lob

Lob brings address verification and direct mail automation together on one platform. Lob’s US Address Verification solution is CASS Cycle O certified, so you can standardize, verify, and enrich address data before a piece enters production.

Use the API to check individual addresses in real time or process lists in bulk. Then turn clean customer data into personalized direct mail without adding another vendor or manual handoff to your workflow.

Book a demo to see how Lob can help you reduce failed deliveries and run direct mail with cleaner data.