The Export MEI plugin uses an external web service to convert a MuseScore score to the Music Encoding Initiative format. Both "full" MEI and the MEI Basic export formats are supported. The conversion to MEI is provided by Verovio (https://www.verovio.org).
By default the plugin uses the conversion service available at https://mscore-plugin.verovio.org/mei as the default. The full source code for the server is available on GitHub, and users are welcome to implement their own service.