The mail will be irradiated using machines ordinarily used to kill bacteria in food and will be returned to Washington for delivery, said Deputy Postmaster General John Nolan.
A Postal Service spokesman said the effort would soon be extended to Florida, New York and New Jersey -- all of which have had mail contaminated with anthrax.
The radiation technology has "never been used for this type of mail processing before," Nolan said.
"However, it's been used extensively in the food industry and other industries for some time, and scientists, technologists tell us that it is absolutely going to be able to do the job for us, to kill bacteria in and on envelopes. Among those bacteria would be anthrax.
"Most mail is not even remotely a problem," Nolan said. "Magazines, financial statements, bills are produced in very tightly controlled environments and are not really the target pieces that we've seen."