1. Copy the plugin: Place the plugin folder in plugins/MultipleTransportBundle
  2. Clear cache:
    php bin/console cache:clear
  3. Reload plugins list:
    php bin/console mautic:plugins:reload
  4. Re-generate assets:
    php bin/console mautic:assets:generate

After installation, go to Settings > Plugins in Mautic and make sure Multiple Transport is enabled.

You can now proceed to configure the plugin settings.