Traditional ACS allows customers mailing letters, flats, or parcels to receive address corrections in an electronic format. It is available for all mail classes, including First-Class Mail, USPS Marketing Mail, Periodicals, Package Services, and Parcel Select. Mailing customers identify their mailpieces with a postal-supplied seven-character alpha identifier known as the ACS participant code. The pound sign (#) delimiter must precede the ACS participant code. In addition, mailer ancillary service endorsements must be printed on all mail other than periodicals. All ACS mail must include a valid domestic return address.
