This PHP function checks if the email address is syntactically correct.

check-email-address.php

function check_email_address($email_address) {
  if ($email_address !== '') {
    $result = filter_var($email_address, FILTER_VALIDATE_EMAIL);

    if ($result === FALSE) {
      return false; // email address is not valid
    } else {
      return true; // email address is valid
    }
  } else {
    return true;
  }
}