Though I don't know the layout of your table mail, if you have an index value on the table (which you should), perhaps something called mail_id, you can do something like this:
$fromuser = $r["fromuser"];
$sub = $r["sub"];
$message = $r["message"];
$id = $r['id'];
$Text = "<a href=CMail.php>$fromuser</a>";
$Text2 = "<a href=readmsg.php?mail=$id>$sub</a>";
Then, inside readmsg.php...
if ( isset($_GET['mail']) && !empty($_GET['mail']) )
{
$string = "SELECT * FROM `mail`WHERE `mail_id`='$_GET['mail']'";
//blah blah
}
Hope this helps.