This plugin sends mail to registered users.
This function will be added in the stable version of plugin.
For sending letters in plugin used wp_cron - WordPress function for periodic execution of any planned actions. This function depends on the traffic of your site: the more visitors, the faster the letters will be sent.
Try to deactivate and activate the version you want to use (either free or PRO).
Please make sure that the problem hasn't been discussed yet on our forum (http://support.bestwebsoft.com). If no, please provide the following data along with your problem's description: