Block specific domain

I have a problem with my blog because a lot of emails are sent to one of these emails (different account some domain):

• gemmasmith.co.uk
• jenniferlawrence.uk
• b-mail.online
• katykahn.co.uk
• sarahconner.co.uk
• marymarshall.co.uk

I don’t understand why the blog is sending all those emails. So, I tried to block the registration for those domains. First, I added in the function.php of the theme the following code

// prevent user registration in wordpress from specific domain
function wpcs_disable_email_domain ( $errors, $sanitized_user_login, $user_email ) {
    list( $email_user, $email_domain ) = explode( '@', $user_email );
    if ( strpos($email_domain, 'b-mail.online') != false || 
		 strpos($email_domain, 'dynainbox.com') != false || 
		 strpos($email_domain, 'gemmasmith.co.uk') != false ||
		 strpos($email_domain, 'marymarshall.co.uk') != false ||
		 strpos($email_domain, 'katykahn.co.uk') != false || 
		 strpos($email_domain, 'roastedtastyfood.com') != false
	) {
        $errors->add( 'email_error', __( '<strong>ERROR</strong>: Domain not allowed.', 'my_domain' ) );
    }
    return $errors;
}
add_filter( 'registration_errors', 'wpcs_disable_email_domain', 10, 3 );

but the emails are still sending. Then, I tried different plug-in like Ban Hammer or WP Security. Nothing.

Could you help me to understand how to block all those emails to these domains?

Thank you advance,
Enrico

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