Area Mail is NTT Docomo's emergency alert broadcast service. It delivers earthquake early warnings and tsunami warnings issued by the Japan Meteorological Agency, along with disaster and evacuation information and special weather warnings issued by government ministries and local authorities, to Docomo devices inside the target area at once. au and SoftBank offer equivalent services under the name "emergency alert mail," which is also the general term used when referring to all of them together.
Technically, Area Mail relies on CBS (Cell Broadcast Service). Whereas ordinary SMS is one-to-one messaging, CBS sends a single message from base stations to every device in the target area simultaneously. Because it does not process recipients one by one, it is far less affected by network congestion during a major disaster, and there is no monthly fee, data charge or information charge for receiving it.
When an Area Mail arrives, the phone shows a pop-up on screen and plays a dedicated buzzer or alert tone. As a rule it sounds even in manner mode, by design, so that life-critical information gets through; exactly how it sounds can differ by handset and settings. It cannot be received in airplane mode or when out of range, however. Devices that are powered off never get it either, and unlike ordinary mail it cannot be retrieved later.
The 2011 Great East Japan Earthquake brought wide recognition of how important Area Mail is. In February 2023 long-period ground motion class was added to the criteria for earthquake early warnings, so cases where class 3 or above is expected are now also covered. Because CBS depends on the handset receiving a broadcast from the base station rather than on the terms of a contract, devices on budget SIMs (MVNO) receive it as well. Overseas SIM-free handsets not sold by a Japanese carrier, on the other hand, fall outside carrier testing and may fail to receive it. It is worth checking once that "emergency alerts" is enabled in your device settings. See the disaster phone guide for more.