The advancements made in mail delivery over the years have allowed for an increasing amount of automation. The United States Postal Service (USPS) has an incredibly complex system to sort mail at fast speeds and without needing a lot of people, as detailed here. (That’s not to say the mail flows automatically — the USPS still employs well over half a million people.) In most cases, machines can route mail faster and with more accuracy than humans can. The Postal Service estimates that 95% of the mail sent through the USPS arrives pre-sorted by machine, only requiring human eyes to inspect the envelopes when the letters move from mail truck to mail box.

But how does the Post Office deal with that last 5%?

There is a warehouse-sized office building in Salt Lake City, Utah called the Remote Encoding Center, or REC. It’s the office for roughly 700 postal workers at any given time, the vast majority of which are situated at computers, looking at the scanned addresses computers can’t discern. It’s a round-the-clock, 365-day-a year effort. Even though it only handles a small fraction of the mail, in 2013, the USPS handled about 160 billion different parcels, according to a New York Times profile of some of the REC workers. Five percent of that is still a lot of letters.

After the computer system shows the envelope to the employee, the employee takes his or her best guess at, well, anything he or she can make out. A number here, a few letters or a word there — the more the better, of course. As the worker fills in clues in hopes of determining the correct address, the computer provides a set of potential matches using the given information. If one looks like a potential match, the worker taps it and, if the computer agrees that the selection fits, that scan disappears — rerouting the actual letter to the place its addressee is located — and the next undecipherable-by-computer address hits the screen. This repeats day in, day out.

Salt Lake City isn’t the only location with a REC — there’s also another one in Wichita, Kansas — and according to the Wall Street Journal, at any point during the day, the two have as many as 1,900 workers translating scribble scrabble into useful addresses. Not all addresses can be deciphered — a letter to “Grandma” without any further information is considered a dead letter and shredded — but the vast majority of the items which pass over the REC monitors end up at their intended destinations.

And it happens rather quickly. REC workers process as many as 20 addresses per minute — that’s 1,200 per hour, around the clock — with exceptional success. The intensity, though, is more than many can handle. The Wall Street Journal further reported that 20% of all REC workers last five or fewer weeks in the job, unable to keep up with the breakneck pace.

Bonus Fact : From 1897 until 1953, New York City had a mail delivery system which used a system of pneumatic tubes. At its peak, according to Wikipedia, 30% of the city’s mail was delivered via the series of tubes.

From the Archives: Post-a-Nut: How to mail a coconut.

Related: “The Post Office Book: Mail and How it Moves” by Gail Gibbons. It’s a kid’s book, but it could prevent future letters from being addressed to only “Grandma” and suffering a sad, unintended fate.