Mailspring plugin. It adds support for Launcher API (Unity) notifications to the dock
It is possible to use this plugin for Dash to Dock extension. Make sure your Dash to Dock build and linux distributive have support for unity notifications. You may need to install a version from the Dash to Dock repository.
git clone https://github.com/snqlby/launcher-api-support.git
cd launcher-api-support
npm install && npm run-script build
Open your Mailspring app and select launcher-api-support directory: Developer -> Install a Plugin