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Postal Address Verification (also known as address standardization, address validation, address verification and CASS certification) is the process used to check the validity and deliverability of a physical mailing address. According to the United States Postal Service, an address is valid (or mailable) if it is CASS-Certified. This means that it exists within the comprehensive list of mailable addresses in their Address Management System. This is different from the credit card Address Verification System (AVS) which is the method used by credit card processors to authenticate ownership of a credit card by verifying that the account on the credit card matches the billing address on file. Credit card AVS does not determine deliverability of an address.
Before an address can be certified as "deliverable" (CASS-Certified) it must first be standardized. Standardization takes an address and converts it to a standard format by correcting the address (if possible) and also adding missing information (such as ZIP codes) to produce a complete address containing a street address, city, state, and ZIP code.
The following methods are used to achieve the most accurate address possible:
USPS approved abbreviations are used whenever possible to clarify and shorten the address.
Because a misspelling will usually result in an undeliverable address, whenever possible commonly misspelled words are corrected. These are typically city names and street names. Due to the large number of possible misspellings, not all of them can be accurately caught and corrected.
Many cities have a number of different commonly used names. Each ZIP Code in the Address Management System has one "preferred" city and may have one or more "acceptable" cities.
CASS-Certified software will accept as standard any "preferred" or "acceptable" city for a 5-digit ZIP Code. Only a city that is "preferred" or "acceptable" will be recognized as standard. These systems will attempt to correct incorrect city names and any that cannot be corrected will be rejected.
An address must be complete in order to be valid. This means that it must have a street, city, state and ZIP code. Whenever possible, addresses that are incomplete will have the missing information added.
If a valid ZIP code is provided but the city and state are missing, the city and state will be added.
If a valid city and state are provided but the ZIP code is missing, the ZIP code will be added.
The standardized address is then compared against the entire list of valid addresses in the Address Management System to determine if it is a valid address. Address validity is based on many different factors including: address renumbering (via the USPS Locatable Address Conversion System) and address completeness. If an exact match is not found an acceptable alternative will be used (if available).
If the address is valid, it will be assigned a ZIP+4 code something like this: 12344-5678, where the first five digits are the ZIP code and the trailing four digits are the delivery range. An address with a ZIP+4 code (or nine-digit ZIP code) is considered to be valid. In most cases, this means that the address is deliverable. However, if the USPS has the address listed as "VACANT", it will not be delivered, even though the address is valid.
The USPS offers address verification directly on their website. Addresses are processed one at a time by typing the address into the provided fields. The USPS also licenses their services to companies that provide the CASS-Certification in bulk. These 3rd party providers typically allow processing of address lists in CSV or Excel format. They may also provide an API allowing the use of address verification services from within a program or website.
Pricing from 3rd party address verification providers ranges from free (for non-profit organizations) to thousands of dollars per year, depending on the number of addresses.