The New York Times reported Saturday that 160 billion letters and packages are photographed each year, and the pictures are made available to law enforcement officials upon request.
Unlike phone records collected by the government, the mail service does not keep the photographs, partly because it would be too expensive to do so, Donahoe said.
Instead, they are discarded after 30 days, the Times said.
"It's done by machine [at 200 processing centers around the country] so there's no central area where any of this information would be," Donahoe said.
"It's extremely expensive to keep pictures of billions of pieces of mail. So there's no need for us to do that," he said.
Opening the mail would require a search warrant, but photographing the outside of letters or packages does not require a warrant.
Law enforcement agencies need only to fill out a form to be given access to a picture, the Times said.
The program is called Mail Isolation and Tracking. It was created in reaction to the deaths of five people from mailed Anthrax in 2001.
Two of the five people killed were postal workers, the Times reported.