If you add a table "signal_emails", the following (entirely untested) code could work:
PHP Code:
$db = mysql_connect(...);
$q = mysql_query("SELECT DISTINCT email FROM signal, signal_emails "
. "WHERE unread = 1 AND destId = signal_emails.steamId", $db);
$rows = mysql_num_rows($q);
for($i = 0; $i < $rows; $i++) {
$res = mysql_fetch_array($q);
mail($res[0], "Unread PM", "You have one or more unread PMs in your inbox!");
}
mysql_close($db);