/**
  * @param  Parser $parser
  * @return array
  */
 private static function getHeadersRelevantForMailerDetectionEml(Parser &$parser)
 {
     return self::filterHeaders(['x-mailer' => $parser->getHeader('x-mailer'), 'message-id' => $parser->getHeader('message-id'), 'received' => $parser->getHeader('received'), 'mime-version' => $parser->getHeader('mime-version'), 'user-agent' => $parser->getHeader('user-agent')]);
 }