Did you ever wonder what happens to mail with an illegible address or that’s sent to someone who no longer lives at the address? The U.S. Postal Service refers to this mail as undeliverable as addressed, or UAA mail. What happens next depends on how the mail was sent, such as the mail class and what services were paid for. It could be forwarded, returned to sender, or treated as waste. While returning the mail to senders allows them to update the addresses, it is costly and inconvenient to both