Domestic postal traffic carried by United States airlines plunged 21.1 percent in January in the latest sign that the air carriers are failing to win back the mail business they have sought to recover.

The domestic decline was the 10th straight monthly slide in the air carriers' postal traffic measured by the Air Transport Association and the 3.1 percent slip in international business meant the airlines hauled 15.3 percent less mail in January than they reported in the same month last year.

The January decline follows a 7.8 percent system-wide decline in postal traffic for the airlines in 2005 and comes as the U.S. Postal Service is revamping its network operations again to make transporting mail more efficient.

The decline in postal business left overall cargo traffic for the airlines up 1.2 percent in January, with freight and express traffic up 2.3 percent.

Domestic traffic was down 0.5 percent, although once mail was removed from the figures domestic freight and express edged up 1.5 percent. International freight and express grew 3 percent over January last year.