In 2010, the United States Postal Service (USPS) reported that about 4.7 percent of mail was not delivered by address (UAA). It is not possible to calculate the percentage of lost letters because the actual number lost is unknown.
It is difficult to calculate any percentage lost. The United States Postal Service does not collect data on the number of items lost; if they could count them, they wouldn’t be lost. The USPS may occasionally misplace, find, and deliver properly addressed mail, but properly addressed items are rarely lost. UAA letters are relatively common, and the USPS has a specific protocol for dealing with UAA letters.
Source : www.gadgetclock.com