This is an Xcode template for Mail.app plugins (mailbundles). The resulting mailbundles work on Snow Leopard (up to 10.6.4, both 32 and 64 bits).
- Download and extract the template.
- Move the entire folder to
/Developer/Library/Xcode/Project Templates/Application Plug-In/
. - Create a new Xcode project based on this template.