Complex sentence: According to the United States Postal Service, an address is valid (or mailable) if it is CASS-certified, meaning that it exists within the comprehensive list of mailable addresses in their Address Management System.
Split into simpler sentences: 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.