If you want to block some email addresses or specific domains, you can use this plugin called Ban Hammer which allows you to block users from registering on your website. You can use this WordPress plugin to prevent unwanted users from registering on your website: Ban Hammer
WordPress backup up plugin that let us schedule a backup of database and files together or separately and set an option to keep certain number of backups on the site. BackUpWordPress https://wordpress.org/plugins/backupwordpress/
Are you looking for a fixed notification bar at the bottom of your WordPress website? Here is a plugin that you can try: WPFront Notification Bar This plugin comes with lots of features and customization and you can have almost all the commonly used static and fixed footer elements. You can display your email address and number or social media links or anything that you like.
Sometimes you may feel like CIA or FBI and may want to check which plugins a WordPress website is using (mostly your ugly competitors). In order to find the list of plugins a WordPress website uses, you can check this website. http://www.wpthemedetector.com/