To reduce the number of spam user account registrations on your child site, it is helpful to verify the validity of emails for those user accounts.
There is a new service and WordPress plugin, which will help reduce those spam account registrations. The plugin is called EVA Email Validation. The plugin requires that you sign up for an account at the EVA site.
With the plugin installed and activated, it will add a new admin menu for:
Settings > EVA – Email Validation
You will need to create a new API key for the child site from the EVA site. Once you have created an API key, you can copy the key into the plugin settings and enable if all forms, comments, and user registration will be EVA email validated.
For the EVA Email Validation plugin only enable the following settings if using WooCommerce on the site.
Verifying that email addresses for user registrations are valid, and not spam (or using disposable or fake emails addresses), will help you focus on dealing with actual users registrations on your child site.