Another Wiimail message header
Posted by bigredpimp on March 14, 2007
X-Wii-AppId: [App ID]
This one apparently is used to send data to specific apps on the Wii, eg. the Mii Channel. The first known use of this one is through NCL (Nintendo of Japan) to send custom celebrity Mii to their users.

So for that case, the message contained the following:
X-Wii-AppId: 0-48414341-3031
More info on the use of this as I look deeper.
