If you’ve ever wondered, “What happens after I send a mailing and some pieces never reach their destination?” — this is for you. Let’s break down ACS™, why it matters, and how it pairs with other tools to keep the mailing machine from leaking money.
Key Highlights:
- ACS™ = USPS feedback loop. It tells you what happened to your mail after it entered the system — forwarded or undeliverable (returned or wasted) and why
- Move update compliance. ACS™ can be used to meet the Move Update requirement on mailings that repeat at least every 95 days to the same addresses.
- Not a replacement for NCOALink®® . NCOALink® updates addresses before mailing; ACS™ provides results after. The two complement each other.
- Cuts waste and cost. By identifying why mail fails, ACS™ helps direct efforts to reduce undeliverable-as-addressed (UAA) mail to save on postage, printing, and handling.
- Data-rich detail. From change-of-address records to Nixie codes (non-delivery reasons), ACS™ provides insights you won’t see elsewhere.
- Multiple variations and formats. Traditional ACS™, OneCode, Full Service (free if you qualify), plus the SingleSource format. All deliver similar feedback at different levels of cost and automation.
- Actionable intelligence. Used well, ACS™ data can tighten list quality, improve compliance, and highlight operational issues that need fixing.
The Basics
Address Correction Service (ACS™) is a post-mailing tool from USPS. You send something, and after mail enters the USPS system, ACS™ gives you feedback: Was it undeliverable? Was there an issue with the Person at that address (the addressee move)? Was there an issue with the Address (missing apartment number, no such street,..)? Was there an issue with the physical mailpiece (Illegible).
It’s not just about “oops, wrong address.” ACS™ returns specifics: reasons (called Nixie codes if not a change-of-address issue), change-of-address (COA) data, even whether an address is temporarily vacant or the current resident indicated the intended recipient is deceased. It’s built for mailers who want to close the loop — know what happened to pieces after they mailed.
Why ACS™™ Matters: The Real Costs of Bad Mail
If you’re treating undeliverable mail as “just part of doing business,” you’re leaving money and insights on the table. Here’s what ACS™ buys you:
- Reduced waste. Fewer pieces sent to bad addresses means fewer resources wasted — printing, postage, handling.
- More accurate customer data. Those COA records and Nixie reasons help clean up your database. Move updates, foreign addresses, temporary moves — ACS™ catches stuff that often escapes pre-mail tools.
- Faster feedback. Rather than learning about mail issues when someone complains (“Hey, where’s my mail?”), or until the physical pieces are returned, ACS™ gives you electronic alerts so you can act sooner.
- Better compliance & credibility. For many mailers (especially big ones), USPS has rules or expectations around move-updates. Using ACS™ helps demonstrate you’re serious about keeping address quality high.
- Operational visibility. Trends, recurring problems (e.g. certain regions, certain address types) can be spotted and addressed.
How ACS™ Fits with Other Tools (Especially NCOALink)
ACS™ isn’t the only game in town. A quick comparison to NCOALink®® helps show where ACS™ shines — and where it doesn’t replace other tools.
| Feature | NCOALink® | ACS™ |
| When it happens | Pre-mailing (before sending) | Post-mailing (after USPS system processes the mail piece) |
| Change of address updates found | Primarily permanent moves (flag foreign moves – some levels flag temporary COAs) | Permanent + temporary COAs, provide new foreign addresses |
| Non-COA issues (“Nixies”) | No | Yes — provides reasons for non-delivery beyond just moved addresses |
| Data freshness | 1-2 weeks old (for service provider licenses) | As current as USPS returns allow (some delay, but quick enough for many uses) |
| Use case | Cleaning your list before you send – meet Move Update Requirement on any mailing | Learning after sending, continuous improvement, address hygiene feedback – meet Move Update Requirement on recurring mailings |
So: NCOALink® cleans ahead of time; ACS™ fills in some gaps and tells you what you missed (or couldn’t see) — or what kinds of address issues you couldn’t avoid.
Key Components of ACS™ You Should Know
Here are the pieces of ACS™ data you’ll deal with — knowing them will help you use ACS™ effectively instead of being overwhelmed.
- COA (Change-of-Address) Records — When someone has submitted a COA, ACS™ can give you their new address (parsed, formatted), move effective dates, etc.
- Nixie / Non-COA Codes — These are reasons mail didn’t deliver that aren’t about COA. Examples include “insufficient address,” “illegible,” “attempted, not known at address,” “vacant,” etc. (Your summary has a more complete list.)
- Intelligent Mail Barcode (IMb) — Critical if you want to tie returned feedback back to individual mail pieces. Without IMb or equivalent identifiers, matching can get messy.
- Delivery Action / Mail Action Codes — Forwarded, returned, waste, unknown, etc. Helps you decide what to do with data.
- Foreign Address Handling — When people move out of domestic USPS coverage, ACS™ can flag and return the new foreign addresses. Useful if you mail internationally or to people who sometimes move abroad.
Versions of ACS™™ & Cost Considerations
Not all ACS™ is created equal. There are different “flavors” or formats — choosing the right one depends on volume, qualification, and how much you want returned, how quickly.
- Traditional ACS™ — electronic notices; some fee per notice. Good for moderate volume or where you want simplicity.
- OneCode ACS™ — uses Intelligent Mail Barcode (IMb); more automated.
- Full Service ACS™ — free (for those who qualify). If your mail volume meets Full Service standards (IMb-compliance and other USPS requirements), this reduces cost and increases speed.
- SingleSource ACS™ — a unified data format that returns records regardless of the ACS™ version used; helps simplify processing.
Check your current USPS service agreements. If you’re sending a lot of mail, qualifying for Full Service ACS™ can shift ACS™ from being “nice to have” to “core.”
How to Use ACS™ Without Spinning Your Wheels
Having ACS™ is one thing. Getting value requires setup and follow-through. Here’s how to use ACS™ smartly:
- Ensure tracking back to mail pieces. Use IMb where possible; keep it unique for a decent period (since returns can lag).
- Segment your data. Look for patterns: which zip codes have high undeliverable rates, what address components (apt number missing, etc.) are causing trouble.
- Incorporate regular feedback loops. Use ACS™ data to feed into your list cleaning, address collection, or purchase processes.
- Validate new vs old addresses carefully. Some ACS™ returns will have “garbage” or questionable new address data — e.g. forwarding address that’s invalid. Use DPV (Delivery Point Validation) or other quality checks. And repeat these checks in case of new construction where the data is still getting into USPS systems.
- Validate the NIXIE reason: for many of the NIXIE reasons, results from other processes (CASS™ and NCOALink®®) can confirm or further clarify the issue with the address and, therefore, the steps needed to correct.
- Train people & systems. Make sure your operators, data team, or whoever handles address entry know about ACS™ codes, their meanings, and what action to take.
7. When ACS™™ Isn’t Enough (But Helps You See What’s Missing)
It’s tempting to think ACS™ solves all undeliverable mail. It doesn’t. Some constraints/gaps:
- If you never run pre-mail list hygiene (NCOALink®® or similar), ACS™ can catch only after the fact.
- Delays happen — mail pieces, COA processing, USPS system delays — so data isn’t always “real time.”
- New address data returned might be messy or incomplete.
- ACS™ doesn’t necessarily fix root causes (e.g. people move, data entry errors, address formatting). It shows you what happened — you still have to build processes to prevent recurring problems.
Why It’s Worth Your Attention
Here’s the deal: ACS™ isn’t sexy. It’s not a dramatic transformation overnight. But it is one of the most underutilized, high-ROI tools in the mailing world. If you want to reduce waste, tighten up data, improve deliverability, and stop “sending money into the void,” ACS™ is a lever worth pulling.
Want To Explore Further?
For more information about ACS™ Processing and Results, request a copy of Adam Collinson’s most recent presentation to the Central Illinois PCC: “ACS™ 101 – The Basics of ACS™ Processing and Results.”
