mepr-restrict-email-domains.php

<?php

function mepr_restrict_email_domains($errors) {
  # Allowed email domains.
  $allowed_email_domains = array('gmail.com', 'hotmail.com');

  $email_address = isset($_POST['user_email']) ? trim($_POST['user_email']) : '';

  # Don't bother validating if the email is empty.
  if(empty($email_address)) {
    return $errors;
  }

  $user_email_array = explode('@', $email_address);
  $user_email_domain = array_pop($user_email_array);

  # Check if the user's email domain is allowed.
  if(!in_array($user_email_domain, $allowed_email_domains)) {
    $errors[] = 'Your email domain is blocked from signing up.';
    return $errors;
  }

  return $errors;
}
add_filter('mepr-validate-signup', 'mepr_restrict_email_domains');