The United States Postal Service says for the first time in history, it will default on a payment into fund for health benefits for future retirees.

Postal officials say they do not have the $5.5 billion owed to the U.S. Treasury Thursday. Officials also say they will likely miss another $5.6 billion owed in September.

Defaulting on a debt will not affect mail delivery or salaries and benefits for current employees. Post offices will be open as usual.

But it does cast more doubt on whether businesses and customers can keep relying on traditional mail service which has served the country for almost 250 years.

The post office gets no taxpayer funds. It relies on the sale of stamps and other products for revenue. It is losing billions of dollars a year as people turn to email to send letters and the Internet to pay bills.

The Senate passed a bill in April to help the post office avoid default. The House of Representatives has yet to act.