code for 5.4, the item in red is whats added, the rest is for context of where to place the code.
Code:
$body = stripslashes($body);
$message = ("
----------------------------
your short message here
http://www.yourwebsite.com/upgrade.php?pflag=retrieve&id=$lid
");
$body = "$body" . "$message";
if (empty($_POST['captcha_key']) || empty($_SESSION['captcha_key']))
please note, the tag remains if someone sends an email to a user as well, to stop this you could do a quick and easy hack of saving the sendmail form above as sendmail1.php and on the detail page change the link to sendmail1.php. this way only the emails to link owners get the tag added... the simple solutions are the least work ! 
Anthony - this is the same as posted above, just clarified it a little. If you cant get it to work, post the sendmail.php file on the forum and i will take a look.
thanks