The earliest underwater mailbox to make its place in the Guiness Book of World Records is located in 10 metres of water in Susami Bay, Japan. In 1999 there was a campaign to promote the Kumano Kodo pilgrimage trail and surrounding areas. The Kumano trail is depicted on a Japanese stamp issued November 13, 1998. Nearby Susami lacked a comparable tourist attraction so Toshihiko Matsumoto, then the postmaster, proposed using an undersea postbox to focus attention on the town. Two old, red postboxes have been altered for the underwater mail collection and are used in rotation every six months, so that they can be protected from corrosion from the seawater by being cleaned and repainted. Hiroaki Yamatani, the manager of a local diving shop sells water-resistant postcards and divers can write messages on them with an oil-based paint marker. Every few days, cards are collected from the underwater post box by an employee of the shop who takes them to the local post office. They usually reach their recipients within a week of being mailed. According to a 2013 report, in each year since the box was established, 1,000 to 1,500 pieces of mail were dispatched from this officially recognized collection point in the local postal system. The post box had its profile enhanced when it was featured in a 2015 commercial for Daihatsu’s Cast Activa automobile. A 2022 report stated that 40 000 post cards had been sent from the mail box.

Tuna, yellowtail and spiny lobsters, all depicted on Japan's fish commemoratives
of 1965-1966, are fished near Susami where the underwater mailbox is located.

A 1990 Japanese prefecture issue for Wakayama depicts the "Old Path of Kumano" which was also
the subject of the promotional campaign leading to the creation of the underwater mailbox.