Christmas is the busiest time of year for both Santa and the United States Postal Service. But while Santa has magic on his side, the USPS must rely on technology to make its deliveries. The service expects to distribute about 15.5 billion pieces of mail during the 2015 holiday season, which is more than 2 times the number of people on Earth.

What with so much mail zipping around the country, odds are some of it will never reach its final destination (fingers crossed that'll include Aunt Gale's ugly Christmas sweater). That's because the service uses computers to route the mail, and about two percent of the time (about 40 million pieces of Christmastime mail), the address on a package is illegible. Bad handwriting, water damage, archaic fonts and those plastic windows on letters all cause trouble for the computers.

That's where Karen Heath and her staff at the Remote Encoding Center in Salt Lake City step in.

"It's the handwriting, like your grandmother's, so unique that the computer has a hard time deciphering it," says Heath, manager at the center.

The U.S. Postal Service has a massive 78,000-square-foot branch, tucked away in the Utah capital, that deciphers illegible addresses. On a normal day, about 5 million pieces of mail are funneled through this branch, but as it creeps closer to December the number can be as high as 11 million, says Heath.

With just under 1,700 employees, the Center employees tackle all of the United State's illegible addresses in 33 different shifts that operate 24/7. And, according to Heath, they have a high success rate.

"We're getting [illegible addresses] from facilities from Hawaii to Puerto Rico and all the way across," Heath says. “Trying to identify what the sender has written is like a puzzle and our [employees] are putting the pieces together.”

When mail enters a regular postal service processing facility, large, powerful machines read the address on the envelope and compare it with a master database. Once there's a match made, the computers print a barcode onto the piece of mail.

If the computer can't read the address because of water damage or your grandmother's ornate script, it sends a picture of the address to a computer at the Remote Encoding Center.

For employees of the center, that means looking at thousands of addresses every day. Even the slowest (and usually the newest) “data conversion operators” can identify about 750 addresses per hour, whereas more experienced employees generally average about 1,600 per hour. "We have to walk a fine line of focusing on accuracy and not speed," Heath says.

That doesn't mean they don't have employees who are lightning fast; the center's quickest employee can decipher 1,869 images per hour. New hires must go through a 55-hour training test that Heath likens to a “Star Trek” exam.

"The training that a new employee gets, it's very intense," she adds. "It makes them fail over and over again. It feels impassable."

These operators don’t guess. The training gives them the expertise to accurately type in addresses that are then checked against the USPS database. Most of the time, there’s a match. When they don’t succeed–the water damage is too severe, the text too illegible or the information too incomplete—the mail goes to the department’s “dead letter” office, officially called the Mail Recovery Center. This is the postal service’s last resort, where employees make one final effort to find addresses by opening mail and examining its contents for clues.

After that, packages that can't be delivered or returned are sold at an online auction, where you can find GoPros, laptops, watches and robotic kits. “Some lots come with unexpected surprises, like $5,000 worth of marijuana hidden in a painting or human cremains mixed in with a collection of tableware,” according to the podcast 99 Percent Invisible.

Any money gets sent to the U.S. Department of Treasury and letters may be recycled into paper, says Lynn Heidelbaugh, a curator at the Postal Museum.

Heath has been working at the center since 1994, when the postal service opened its first illegible mail processing facility in Utah. Before the advent of computer programs, letters were sent to the “dead letter office” where employees investigated each piece of mail in a slow, painstaking process. USPS expanded its operations, peaking at 55 facilities like the one in Utah.

But by 1998, computer technology produced by the likes of Siemens and Lockheed Martin had surpassed human capabilities for speed, and, today, all but the Utah facility has shuttered. Engineers for these companies have been updating this technology constantly over the past few decades, fulfilling government contracts worth hundreds of millions of dollars in some cases.

"The number of items that [are illegible] has been diminishing over the years because the machines have gotten better at reading and matching [addresses]," says Nancy Pope, a curator at the Smithsonian National Postal Museum. Eventually, even the Remote Encoding Center could close.

If you're concerned about getting mail to your loved ones, the postal service recommends addressing all post with a sans-serif font, point size 10-12. But if you're set on writing all your mail by hand, don't worry, Heath's team has got your back.

"It's fun to know that you're getting somebody's package to them," Heath says. "There's a piece of mail that's not going to get to where it needs to go unless [we] invest something of [ourselves] in making sure that happens."