Is it meant to be? It's just an identifier that you can use for mail filtering if you so wish as far as I am aware.
/**
* Generate a unique email Message-ID using the moodle domain and install path
*
* @param string $localpart An optional unique message id prefix.
* @return string The formatted ID ready for appending to the email headers.
*/
function generate_email_messageid($localpart = null) {
global $CFG;
$urlinfo = parse_url($CFG->wwwroot);
$base = '@' . $urlinfo['host'];
// If multiple moodles are on the same domain we want to tell them
// apart so we add the install path to the local part. This means
// that the id local part should never contain a / character so
// we can correctly parse the id to reassemble the wwwroot.
if (isset($urlinfo['path'])) {
$base = $urlinfo['path'] . $base;
}
if (empty($localpart)) {
$localpart = uniqid('', true);
}
// Because we may have an option /installpath suffix to the local part
// of the id we need to escape any / chars which are in the $localpart.
$localpart = str_replace('/', '%2F', $localpart);
return '<' . $localpart . $base . '>';
}