Thunderlink is a fantastically useful add-on for the Thunderbird mail user agent. It lets you create links to email that you can put into all sorts of documents (like ToDo lists or notes). The links use the URI scheme thunderlink that needs to be handled by whatever mechanism you use to open URIs. On macOS, one way of registering this URI scheme is to use an application like SwiftDefaultApps. It’s as easy as installing the preference pane, adding the URI scheme and associating it with Thunderbird.app.