Block WordPress user account registration by e-mail domain

block-user-registration-by-mail-domain.php

<?php 

/**
* Plugin Name: Block WordPress user account registration by e-mail domain 
* Plugin URI:  https://gist.github.com/adczk
* Description: Block WordPress user account registration by e-mail domain
* Author:      adczk
*
* Author URI:  https://gist.github.com/adczk
* License:     GPLv2 or later
*
* Use as MU plugin; config in code comments
*
*/

function wpmu_stop_registration_by_domain( $errors, $sanitized_user_login, $user_email ) {

	// below you list email domains to be blocked
	// comma-separated
	$blocked_domains = array(
		'emaildomain.com',
		'otherdomain.net', 
	);
	
	// custom error message
	$error_msg = "You're not allowed to register!";

	foreach ( $blocked_domains as $key=>$value ) {
		
		if ( strpos( $user_email, $value ) !== false ) {
			$errors->add( 'banned_domain_error', $error_msg );
		}
	
	}

    return $errors;
}

add_filter( 'registration_errors', 'wpmu_stop_registration_by_domain', 10, 3 );