The USPS provides free Early Warning System (EWS) data to prevent matching errors due to the age of the address information in the .gsd files. The USPS creates a new EWS dataset each week that you may download from the USPS website (https://postalpro.usps.com/cass/EWS).
The USPS requires EWS files for CASS™ processing. You can use the use.gsd and usw.gsd files (located in the Spectrum Geocoding Data Package) for 135 days, but after 105 days you cannot create a CASS report.
When you download the EWS file, place the file in the same directory path as the .gsd data and make sure to name the file EWS.txt.