Margaret Holloway
Postal standards, CASS and bulk mail for sthan.io
Margaret writes about the postal rules behind US addresses: CASS certification and when you need it, PS Form 3553, ZIP+4 codes, and the move away from the retired USPS Web Tools APIs.
She cites the rule itself (the Domestic Mail Manual, the CASS Technical Guide, USPS release notes) with the date it was read, so you can check it at the source.
- 5 articles
- 37 minutes of reading
- Sources dated in every article
- Latest: Oct 7, 2026
What Margaret covers
- Do you need CASS? Do You Need CASS Certification for Address Validation?what the postal rules say, by use case
- Certified or not CASS-Certified vs Non-Certified Address Validationa side-by-side view and when each is the right tool
- CASS in depth USPS CASS Certification: Complete Guide for 2026how CASS works, its data elements, and postage savings
- ZIP+4 ZIP+4 Lookup API: Get the 4-Digit Add-On by APIwhat the four digits mean and how seven APIs return and price them
- USPS API changes USPS Web Tools API Retirement: What to Move Towhat changed on January 25, 2026 and where to move
Where to start
A reading order for someone who mails to US addresses or calls USPS APIs.
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Start here: what the postal rules require, by use case.
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If you might, compare the two side by side.
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For bulk mailers who need the full picture.
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ZIP+4 Lookup API 7 minWhen you need the full nine-digit ZIP for an address.
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If your code still calls the old USPS Web Tools.
Questions these articles answer
Each question links to the answer inside the article.
CASS
- Is CASS certification required for an address validation API? Do you need CASS
- What does CASS actually certify? Do you need CASS
- What is PS Form 3553? Do you need CASS
- Is CASS-certified validation more accurate? Certified or not
- Can I use both? Certified or not
- What is the difference between CASS and NCOALink? CASS guide
- Is sthan.io CASS-certified? Do you need CASS
ZIP+4 and DPV
- What is a ZIP+4 code? ZIP+4 lookup
- Is there a free ZIP+4 lookup API? ZIP+4 lookup
- Why do I need the unit number to get the right ZIP+4? ZIP+4 lookup
- What is DPV and why does it matter? CASS guide
USPS APIs
- When did USPS retire the Web Tools APIs? Web Tools retirement
- Do the old Web Tools APIs still work? Web Tools retirement
- Is the new USPS address API free? Web Tools retirement
- Can I use the USPS Addresses API to validate signup or checkout addresses? Web Tools retirement
- What is the USPS API rate limit? Web Tools retirement
What these articles rely on
- Postal rules
- Domestic Mail Manual 602 (pe.usps.com)
- CASS
- PostalPro CASS certification page and CASS Technical Guide, Cycle O; PS Form 3553, September 2022 edition
- USPS APIs
- The Web Tools retirement notice, migration FAQ and 2026 release notes; developers.usps.com terms updated January 22, 2026
- Other vendors
- ZIP+4 pricing pages for seven APIs
- Read dates
- Sep 27 to Oct 7, 2026, listed at the end of each article
Related on sthan.io
Products
Terms explained
All articles by Margaret
ZIP+4 Lookup API: Get the 4-Digit Add-On by API
What the four extra ZIP+4 digits mean in the postal rules, three live responses that return them, and how seven APIs return and price ZIP+4.
USPS Web Tools API Retirement: What to Move To
USPS retired Web Tools on January 25, 2026 but kept patching it. The new Addresses API is free only for USPS shipping. What changed and where to move.
Do You Need CASS Certification for Address Validation?
CASS is required to claim automation and carrier route postage prices. What the postal rules say, by use case, and when a non-certified API is fine.
CASS-Certified vs Non-Certified Address Validation
One unlocks postage discounts through PS Form 3553. Both can clean your data. A side-by-side view and when each is the right tool.
USPS CASS Certification: Complete Guide for 2026
Everything you need to know about CASS certification, postal discounts, and how to save up to 40% on postage.