Check if the submitted email on the register process is valid. Runs right before registering a user.

is_valid_email_domain.php

<?php
/**
 * Check to ensure the users' email address matches the allowed list. 
 * Update the $valid_email_domains array to a comma separated list of string email addresses.
 * 
 * @since 1.0.0
 */
function is_valid_email_domain($login, $email, $errors ) {

  	$valid_email_domains = array("imageproperty.com.au","stafflink.com.au");// whitelist email domain lists
  	$valid = false;
  
	foreach( $valid_email_domains as $d ) {
  		
		$d_length = strlen( $d );
  		$current_email_domain = strtolower( substr( $email, -($d_length), $d_length));
  		
		if( $current_email_domain == strtolower($d) ) {
  		
			$valid = true;
  			break;
  		}
  
	}

  	// if invalid, return error message
  	if( $valid === false ) {
  		$errors->add('domain_whitelist_error',__( '<strong>ERROR</strong>: Sorry, your email address is not on the approved list. You must use a valid Image Property email address.' ));
  	}
 }
 add_action('register_post', 'is_valid_email_domain', 10, 3 );