A mail brick is caused when a user has too much mail in their Message Board for the Wii to handle. The Wii will crash upon booting through normal means. However, the System Menu can still be used through Maintenance Mode.
Fixing this brick
This brick can be fixed by entering Maintanence Mode, which is done by holding + and - on the health screen when pressing A. From here, the mail can be deleted using homebrew launched from the Homebrew channel. If the Homebrew channel is not installed, then str2hax can be used for versions 2.0-4.3, or Bluebomb for other versions. Once the HackMii Installer is launched this way, HBC can be installed, and a mail deletion tool can be loaded.