A peculiarity of the building is that it was built using air rights over railroad tracks that terminate several blocks to the north, at Union Station, and so it has no basement. In addition, the Congress Expressway literally passes through the structure. The two-story-high tunnel carries six lanes of traffic.

The building is often described as the world’s largest post office. At its peak, 5,000 workers processed more than 35 million letters annually, using 10 miles of conveyor belts and 48 elevators. Every day, more than 125 trains and 6,000 trucks arrived at the facility.

Gradually, however, the building became obsolete as the mail-sorting process was automated. “The column spacing and the ceiling height do not work for automated operations,” Mr. Samra said. “Automated operations require a more horizontal building.”

In 1996, it was replaced by a much smaller building  500,000 square feet  a block south. Still, there is considerable nostalgia for the old facility.

“I miss the grandeur of the lobby,” said Musette Henley, who worked in the building in a variety of jobs from 1961 until its closing day and is now a customer relations representative in the new facility. “They don’t build buildings like that anymore.”

The imposing Neoclassical lobby at the north end of the building, which has cream-colored marble walls and an elaborate inlaid marble floor, is certainly a stunner: 340 feet long and 40 feet wide, with a towering 38-foot ceiling.

Image Rick Levin, president of a local firm that will conduct the auction, acknowledged that in the building's current state, it represents a significant challenge to developers. Credit... William Zbaren for The New York Times

For more than a decade, the Postal Service has been working with Walton Street Capital, a Chicago-based real estate investment firm, to find a new use for the building, with a notable lack of success. “There have been many iterations of redevelopment possibilities, and market demand has come and gone,” said Raphael Dawson, a principal of the firm.