The U.S. Postal Service handles half a billion letters a day. If you started counting right now and did nothing else, it would take you 16 years to reach half a billion. The size and the shape of the envelope and the way it's addressed are important.
Machinable mail needs to be the right size and shape to pass easily through the equipment.
Readable mail must be quickly and accurately read, coded, and sorted by the equipment. That means the address should be typed, computer generated, or neatly printed in the recommended format with a minimum of three lines, aligned at the left. The OCR best reads plain-style, capital letters on white envelopes. Speedy conveyor belts move each piece of mail by the OCR's scanner, which reads the delivery address. Then the OCR's printer sprays on a bar code for that address or reads the bar code applied by a business customer before the envelope was mailed.