The announcement comes a day after regulators refused to allow the post office to raise the cost of mailing a letter by 2 cents.
Postmaster General John Potter says the post office is still considering whether to appeal that decision or make a new rate hike request.
The latest loss is a bit less than the $7 billion the post office had been expecting. But it's on top of a $3.8 billion loss the year before.
Mail volume is down 20 percent over the past two years due to the recession and more communications moving to the Internet.
Potter plans to keep pushing in Congress to reduce mail delivery to five days a week and to shutter thousands of post offices.