Do You Need CASS Certification for Address Validation?
Only for one job: claiming discounted postage on bulk mail. Here is what the postal rules actually say, and when a non-certified API is fine.
CASS certification is required only when you use address results to claim automation or carrier route postage prices on bulk mail. The postal rules (Domestic Mail Manual 602, section 9) tie CASS to those price claims. For checkout forms, CRM cleanup, fraud checks and parcel shipping, no rule we found requires a CASS-certified API.
CASS stands for Coding Accuracy Support System. It is a test the Postal Service runs on address matching software. Pass it, and mail coded by that software can qualify for lower postage.
Do you need it? By use case
| What you are doing | CASS needed? | Why |
|---|---|---|
| Bulk mail at automation prices | Yes | DMM 602.9.1.2: the list must be "matched and coded with CASS-certified address matching methods" |
| Bulk mail at carrier route prices | Yes | DMM 602.9.3.1 and a PS Form 3553 for each mailing |
| Commercial mail, ZIP Code accuracy rule only | Not always | DMM 602.6.2 also accepts other approved methods, including the postal website for small lists |
| Checkout and signup address forms | No rule found | The CASS rules we read apply to postage price claims |
| CRM and database cleanup | No rule found | Same: no postage claim involved |
| Parcel shipping labels | No rule found | Some carriers use CASS data by choice, not as a requirement on you |
We read the rules on September 27, 2026. We did not find a postal statement that says in so many words "CASS is not required for checkout." What we found is that every CASS requirement is written as a condition for a postage price.
What CASS certifies
- The software, not you. Vendors submit their matching software to a postal test file.
- Pass marks are strict. At least 98.5% on ZIP+4, carrier route, five-digit ZIP and LACSLink, and 100% on delivery point coding and DPV (the check that a delivery point exists).
- Two-year cycles. Certification lasts until the end of the current two-year cycle. Cycle O expires July 31, 2028.
- It bundles more than matching. Cycle O certification includes DPV (or DSF2), LACSLink for rural route renames, and SuiteLink for business suite numbers.
- Data must be current. Mail must be coded with a monthly data release that is still inside its permitted window.
The paperwork: PS Form 3553
PS Form 3553 is the CASS Summary Report. You submit one with each mailing claimed at automation or carrier route prices, and keep it for one year. It must be printed by CASS-certified software, and the software must refuse to print it for a non-certified setup. That form is the real reason CASS matters: without it, no discount.
When a non-certified API is fine
If you are not claiming discounted postage, judge an API on what it does for your data:
- Does it tell you whether the address is deliverable?
- Does it catch a missing or invalid apartment or suite number?
- Does it return a standardized address with ZIP+4?
- Does it refuse to guess when an address does not exist? See how to test an address API's accuracy.
- What does it cost at your volume? See how address verification APIs are priced.
Where sthan.io fits. sthan.io is not CASS-certified and cannot produce a PS Form 3553. Use it for checkout, signup, CRM cleanup and parsing messy addresses. For discounted bulk mail, run the final list through CASS-certified software.
Frequently Asked Questions
Sources
- Domestic Mail Manual 602, sections 5, 6 and 9 (pe.usps.com), read September 27, 2026
- PostalPro CASS certification page and CASS Technical Guide, Cycle O (postalpro.usps.com), read September 27, 2026
- PS Form 3553, September 2022 edition (about.usps.com), read September 27, 2026
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