OutgoingEventManager::getId

private function getId($headers): ?int
{
	foreach (explode("n", $headers) as $header)
	{
		if(mb_strpos($header, ':') !== false)
		{
			[$headerName, $headerValue] = explode(':', $header, 2);
			if(mb_strtolower($headerName) === 'content-id')
			{
				$part = explode(':', $headerValue);
				return (int)rtrim($part[1], '>');
			}
		}
	}

	return null;
}