An automated sorting machine in Capitol Heights, Md. Robyn Beck/AFP/ Getty Images

So with all of that manpower and infrastructure, what actually happens when you drop a letter into a mailbox? Here is a brief rundown:

You've properly addressed your letter and added the appropriate postage, so you place it into your mailbox. (You could also drop the letter into a public collection box or take it directly to your local post office.)

A postal carrier collects your letter from the box along with the rest of the mail and takes it to the post office. There, all of the mail is placed on a truck and taken to a mail processing plant.

At the mail processing plant, machines separate mail by shape and size. They also orient the packages so their addresses are right-side up and facing the same direction. Your letter gets its postmark, and machines print cancellation lines across postage stamps to prevent them from being reused.

A unique fluorescent bar code is imprinted on the back of each piece of mail. An optical scanner scans the address, and then a bar code representing the specific address is sprayed on the front of the envelope. If the scanner can't read the address, the letter is manually sorted.

Other processing machines read the bar codes and direct the letters into bins based on ZIP codes — this indicates the next processing plant, in the region where the letter will ultimately be delivered. (Each post office is served by a mail processing plant.) From the bins, the letters are sorted into trays by ZIP code and flown or trucked to the next processing.

At the final processing plant, sorting machines read the bar codes and sort the letters by carrier and into delivery order for that carrier.

The letters are taken to the individual post offices, and the carriers load the trays into their individual vehicles for final delivery.

To take advantage of automation and its optical scanners, the USPS has guidelines for addressing letters. Your letter won't be thrown out if you don't follow the guidelines, but using them can speed up delivery.


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