Only allowing some email addresses to create an account

Just copy the above code and paste it into your theme’s functions.php file.
Here I am going to show you the code which will reject registration from all others email addresses and Only allowing some email addresses to create an account. See the code below

function is_valid_emails($login, $email, $errors ){
     $valid_emails = array("[email protected]");
     $valid = false;
     foreach( $valid_emails as $valid_email ){
         $valid_emails_list = strtolower($valid_email);
         $current_email = strtolower($email);
         if( $current_email == $valid_emails_list ){
         $valid = true;
            break;
         }
     }
     if( $valid === false ){
        $errors->add('domain_whitelist_error',__( "<strong>ERROR</strong>: you can't register email because only allow some specific email addresses" ));
     }
    }
    add_action('register_post', 'is_valid_emails',10,3 );

You can add more email addresses by adding more inside the code:

$valid_emails = array("[email protected]","[email protected]");