Email All Users is a plugin that creates a widget that allows you to send mass email to all registered users in your wordpress site. You configure the widget with a title and mailto-link-title. The widget will then create a mailto:link that will include all users that is active. If you have the plugin «Email Users» installed it will only include users that has approved mass email in the settings.
Te puedes descargar este plugin para utilizarlo en tu instalación autoalojada de WordPress.