Message-ID-url aims to be an implementation of the "mid" URL scheme described in RFC 2392.
In order to find the corresponding message, the application can be configured to look in multiple sources. The first source should be the mail store associated to the mail user agent of the user but other sources can be external services like gmane, or even the Message-ID redirector provided by Debian on http://lists.debian.org/_MID_
A plugin based architecture should allow to add support lots of MUA and lots of external sources so that the user can pick those that are relevant to him. In the default configuration, the software should try to guess what sources to use.
For best results, and avoid some useless redirections through a browser, it is recommended to register the "x-scheme-handler/mid" MIME type in the "[Default Applications]" section of ~/.config/mimeapps.list and associate it with "open-msgid.desktop".
$ head -n 2 ~/.config/mimeapps.list
[Default Applications]
x-scheme-handler/mid=open-msgid.desktop
You can double check that the association is properly recorded with this command:
$ xdg-mime query default x-scheme-handler/mid
open-msgid.desktop