code to validate email addresses (via MX address & PHP mail well-formedness rules)

email_validate.php

/**
 * Implements hook_user().
 * Does advanced email validation.
*/
function revolven_user($op, &$edit, &$account, $category = NULL) {
  if($op == "validate") {
    if($error = adv_email_validate($edit['mail'])) {
      form_set_error('mail', $error);
    }
  }
}

// Sets error for invalid emails
function adv_email_validate($mail) {
  $valid_email = wellformed_email_address($mail);
  if($valid_email == FALSE) { return t('The e-mail address %mail is not valid.', array('%mail' => $mail));
  }
}

/** Checks if an email address is well-formed
 *  and MX record is present.
 *  Requires PHP 5.2 and checkdnsrr() function.
*/
function wellformed_email_address($mail) {
  // simply return TRUE if PHP version is too low
  $version = version_compare(phpversion(), "5.2");
  if($version < 0) {
    return TRUE;
  }
  // also return TRUE if checkdnsrr does not exist
  if(!function_exists("checkdnsrr")) {
    return TRUE;
  }

  if(filter_var($mail, FILTER_VALIDATE_EMAIL)) {
    $form_valid = TRUE;
  }
  else {
    $form_valid = FALSE;
  }

  list($username, $maildomain) = split("@", $mail);
  if(checkdnsrr($maildomain, "MX")) {
    $domain_valid = TRUE;
  }
  else {
    return FALSE;
  }

  if($form_valid && $domain_valid) {
    return TRUE;
  }
  else {
    return FALSE;
  }
}