The United States Postal Service has been an integral element of the nation since the American Revolution. Its job has always been simple yet complex: deliver the mail in a timely and cost-effective manner.

Pick-up and Postage

Whether it acquires mail from a private mailbox, a USPS drop box, or from customers dropping mail off at the Post Office, the postal system begins to work when the USPS takes charge of the mail. It first checks that the address is in a proper format and that the fee for mailing the item, called postage, has been prepaid.

Addresses and ZIP Codes

Mail generally requires a 3-line address, including the name of the person or coampany to which it should be delivered, the street address, and the city, state, and the ZIP Code. Arguably the most important element of an address, ZIP Codes assist in sorting and directing mail. The basic format, introduced in 1963, includes five digits, but in 1983 a new +4 digit format was added to provide more specific delivery data. The original five-digit format, though, is sufficient to ensure a timely delivery. Mail without a ZIP code in the address is separated for manual entry of the ZIP Code by a clerk who looks it up.


The cost of mailing an item, called postage, is prepaid and evidenced by a stamp affixed to the item. The actual amount depends on the item's weight, type and delivery requirements. For example, the standard first class rate for the typical letter was established at 49 cents in early 2014. Items with improper postage are returned to the sender, if possible.

Undeliverable Mail

If the USPS determines that the postage is insufficient or that it cannot deliver an item to the address noted, it attempts to return the item to the sender. For this reason, senders are requested, but not required, to put their own addresses on items they mail, usually in the upper left-hand portion of an envelope.


After accepting the mail and checking the address and postage, mail is then sorted for shipment. This sorting is done by a Multi-line Optical Character Reader, which extracts zip code data from the address, prints a bar code onto the item, and then routes it for further sorting or shipment. Mail generally goes through several sorts.


In its final sort in the post office responsible for delivering it to the addressee, mail is given to a mail carrier, who delivers it to the addressee's mailbox at home. For customers who have a post office box, mail is placed in the box for pickup at the customer's convenience.