It would not matter - forum search does not work properly. At the momemnt You can only google for items & info that You search in this forum
German: Internetoptionen (Internet options), in Tab Programme (programs).
System part of the internet settings, a default handlers for several jobs can be set. The same tab where you can set IE as default.
You can probably also set it in options of the program that You are using.
I think mailto: is a protocol and a default handler is set via registry.
I am not sure because my mail program asked me during install and I chose it as default.