UNDATED (CNN) -- The U.S. Postal Service's financial situation is looking even grimmer.

By Sunday, the service is required by law to pay the federal government $5.6 billion to fund health care benefits for retirees. But it won't do it, because it doesn't have the money.

It will be the second time that the service has defaulted on a payment. The first was last month.

For now, the cash shortage won't affect your mail.

The Postal Services say employees and suppliers will still get paid.
But it says the Postal Service will start running out of money by the spring unless Congress acts soon.

And that could mean drastic cuts to the mail delivery system.