WASHINGTON -- The Postal Service is a giant delivery machine. It bridges the physical and digital using any combination of humans and robots that works. The USPS delivers half the world's mail -- that's 563 million pieces each and every day of 2010. To do so, it employs 574,000 people and 10,000 pieces of mail-sorting equipment. If the Postal Service were a company, it would have been number 29 on the 2010 Fortune 500 list and Benjamin Franklin would have been its first CEO.

Think of all the problems that the USPS has had to solve to become and remain this massive country's only completely inclusive point-to-point network. And they did it under all kinds of different regimes. Horse and carriage? No problem. Railways? Sure. Exploding suburbs and road mileage? Great. This is an agency that saw the introduction of all kinds of competing communications systems: the telegraph, the telephone, radio, television, UPS, and fax machines. And through it all, the volume of mail just kept going up, year after year, for 200 years.

This history is highlighted by a new exhibit at the National Postal Museum, Systems at Work. Any mail network relies on a system of people, machines, and shared information to get work done. And looking at the number of changes that have occurred over the two centuries, I find the Postal Service's ability to keep up astonishing.

Just one example: The Postal Service's OCR machines correctly translate the chickenscratch that passed for an address on 93 percent of hand-lettered envelopes. The rest of the most-excellent machines that the USPS has used through time are housed in the slideshow below.

Despite these successes, there have been some hard times for the Postal Service. The biggest crisis USPS faced probably came in the mid-1960s. During that time, which was before Richard Nixon signed a bill that made the service "self-funding," the Post Office could not get enough funds from Congress to buy the machines they needed to keep up with the post-War explosion in the mail. In October of 1966 the situation came to a head, when, as the museum exhibit put it, "a flood of holiday advertisements and election mailings choked the system." The Chicago Post Office, the largest in the country, "stopped delivering mail for three weeks."

Automation was the only way out. Zip codes, which were only introduced in 1963, became the linchpin in the automated postal system. Imagine life without them: a single person can't sort more than a letter a second, which is at best, 3,600 letters an hour. With the help of machines, postal workers could gain almost an order of magnitude of speed, sorting 30,000 letters an hour. Also, zip codes formed the occasion for this wonderful music video about the introduction of the system. I believe it reaches Pynchonian heights of lyrical genius.

Despite this impressive legacy, the Postal Service is now back on the ropes. While there are a lot of reasons, the key long-term challenge is simple. For 200 years -- TWO HUNDRED YEARS -- the volume of mail that the postal service had to deal with grew. Then in 2006, the annual volume of mail flowing through the system peaked (though, as noted below, it grew into 2007 before starting to decline).

"There have been enormous challenges in the past, I don't want to downplay that at all, but this is the biggest challenge. It's bigger because it's so different. It challenges the one thing that has always been true: mail volume goes up," Postal Museum curator Nancy Pope said. "Mail peaked in 2007, and now for the first time, they're looking at what happens when there is less mail. And nobody at the Postal Service is ready for that. It's just mindblowing. Everyone has grown up with this idea. 'Mail volume goes up.' If you think something bad is coming down the road in five years, a mail price increase will solve it because mail volume goes up."

At the same time as mail volume is decreasing, people still want the ability to receive mail at any time and at any address they choose. As a result, the number of individual delivery points increased by 735,779. That is to say, the costs of maintaining the ability to distribute mail are going up, even though the volume of mail (i.e. revenue) is declining. The USPS has massive fixed-infrastructure costs built into its core as a national service committed to serving everyone. 

That's why the decline in mail volume, spurred by the availability of that other point-to-point communications network, the Internet, is an existential crisis. Maybe one of the two big pieces of legislation ham-tying the USPS -- the 1970 Postal Service Reorganization Act and the Postal Act of 2006 -- will get changed and the immediate crisis for the USPS will abate. But in the long term, the Postal Service has got to deal with its revenues and its costs running in the opposite directions.

The baseline trend has shifted beneath the Postal Service, and like many 20th-century institutions, it has to shift its top priority from speed to resilience. I think the new challenge won't be to deliver mail as quickly as possible, but to preserve the core network of infrastructure necessary to be able to deliver mail to any address. And this time around, more machines and computers won't be the answer.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.