Undeliverable mail is mail that USPS cannot deliver. There are a ton of reasons why mail can be undeliverable.
Mail can be considered “undeliverable” for several reasons. Mail not reaching its intended recipient is usually because the address is incomplete or incorrect. It can also be due to not having a mail receptacle on the property, no apartment number included in the address, or being vacant.
Fun fact: The USPS has refused to deliver mail to homes with loose animals on the property.... so make sure your dog Max is on a leash when your postal worker comes around. 😉
The best way to reduce undeliverable mail is to keep your data up-to-do by Skip Tracing and practicing good list hygiene.