This plugin if enabled, will delete emails older than certain number of days (by default 90 days) from emails log. That helps reducing number of emails stored in database in case you have many emails sent from system (so save your hosting space)
To have system send registrants to administrators email, you need to go to Extensions -> Plugins, find and publish System - OSB Booking Clean Email Logs plugin. It has following parameters, you can review and change it to meet your own need if needed