In 2010, the United States Postal Service (USPS) reported that about 4.7 percent of mail was undeliverable as addressed (UAA). It is impossible to calculate the percentage rate of lost mail because the real amount lost is unknown.
It is difficult to calculate the percentage of anything lost. The United States Postal Service does not collect data on the quantity of items lost; if they could count them, they would not be lost. The USPS may occasionally misplace, find, and deliver correctly addressed letters, but properly addressed items are rarely lost. UAA mail is relatively prevalent, and the USPS has specific protocols in place for handling UAA mail.