To Japanese, having letters delivered on time has come to be regarded as a virtual birthright.
But they are also prolific correspondents - and the thought of all 118 million of them putting pen to paper on the same day is the stuff postmen's nightmares are made of.
Already handling an average 30 million pieces of mail a day - a volume second only to the United States - the Japanese postal service prides itself on leading the world in efficiency.
It has to be that good.
According to postmaster Rinzo Kochiya, whose Yokohama sorting office routinely handles more than 1 million items a day: ''If we didn't clear it all up immediately, we'd soon drown in mail. And, anyway, the public automatically expects a good service.''
The answer is more and more machines.
In major urban centers like Tokyo and Osaka it is possible for a letter to be untouched by human hand from the time it is posted until the local postman drops it through the letterbox at the other end.
Post offices have banks of machines that accept punched-in instructions for special handling - such as express delivery or registered mail - before proceeding to automatic weighing and scanning of letters, calculating the postage, putting on the stamps, and delivering change.
In the downtown Tokyo post office of Harumi, there is a sorter who has demonstrated on national television his ability to handle mail faster than any machine.
Letters fly out of his fingers in a constant blur to the right pigeonholes. It's all very impressive - but he is an exception. In the end, say postal service authorities, round-the-clock computerized sorting operations are the only answer to the ever-growing flood of mail.
At the end of May, the most advanced sorting system in Japan - and in the world, as far as Japanese officials know - was installed in a gleaming new center in Yokohama, serving not only the busy Tokyo Bay port but also the populous Kanagawa prefecture.
The two-way flow of mail through the center currently averages 1.1 million items a day, most of which, with the major exception of parcels, is handled by computers. Sixty percent of this passes through in a hectic seven hours from 6 p.m to 1 a.m.
Yokohama got the first system - two years in developing by Toshiba at a price of almost $8 million - because it had outgrown its original building and a planned move allowed the system to be easily installed from scratch.
Other post offices eventually will follow. The Finance Ministry is even considering the system to sort out returned paper currency from banks to eliminate damaged and counterfeit notes before neatly parcelling them up for recirculation.
The computers have eliminated 100 sorting jobs so far. But a fair number of humans are still necessary because even computers cannot yet do everything.
The Japanese postal service is trying to improve speed and efficiency of the automation through standardization of envelope sizes and use of postal codes. Not everyone cooperates.
When the raw mail drops onto conveyor belts, a computer scanner quickly eliminates mail without a postal code, or where this is written incorrectly or illegibly. These letters are then sorted by hand for insertion into the system further down the line.
With machine-gun rapidity, the automatic sorter spits out letters and cards into banks of small boxes for each of the 950-odd postal districts. Eventually these are automatically tied into neat bundles and marked with a line code. The computer reads the code and routes the bundles onto a conveyor belt of miniature tip-up carts that deliver them to the gaping mouth of the correct mailbag or box.
The bags are hooked onto an overhead line and directed by a code to mail vans waiting outside. The boxes run on rails under the machine's direction in the same way.
Remote control television cameras monitor every stage of the sorting process over several floors. The pictures they send back to a central control room along with information fed by the computer to a giant electronic board allow staff to instantly spot and deal with problems.
Postmaster Kochiya is enthusiastic about his new machine: ''Apart from eliminating 100 sorting jobs, it has made the work lighter and easier for everyone. The system hasn't been fully tested yet, because it has the ability to handle three times the present mail volume, around the clock throughout the year.''
The machines, however, are merely eliminating delays at the sorting stage, not necessarily speeding up the progress of a letter. And, due to staff shortages and union demands, actual mail deliveries have been cut from two to one a day, and only surcharged letters get delivered on Sunday now.