According to the United States Postal Service (USPS), the amount of mail that was undeliverable as addressed (UAA) was near 4.7 percent in 2010. The percentage rate of lost mail cannot be measured, because the amount actually lost is unknowable.
Calculating the percentage of anything lost is problematic. The USPS does not have methods for collecting data on the number of items lost; if they were able to count them, the items wouldn't be lost. Properly addressed letters may be occasionally misplaced, found and then delivered, but properly addressed items are rarely lost by the USPS. Mail that is UAA is fairly common; the USPS has detailed plans in place for the handling of mail that is UAA.