ACS (Address Change Service)
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A USPS service that provides mailers with electronic notifications when mail is forwarded, returned, or undeliverable due to an address change.
ACS (Address Change Service) is a USPS program that gives mailers electronic feedback about undeliverable mail. When a mail piece cannot be delivered (because the recipient has moved, the address is vacant, or the recipient is unknown), ACS generates an electronic notification instead of physically returning the piece.
ACS notifications include the old address, new forwarding address (if available), and a reason code (moved/no forward, refused, vacant, deceased, etc.). This data is delivered electronically through the Intelligent Mail barcode system and PostalOne!.
Compared to NCOA (which proactively updates addresses before mailing), ACS is reactive — it provides feedback after a mail piece encounters a delivery issue. The two services are complementary: NCOA catches moves before mailing, and ACS catches issues that NCOA missed.