ok, so this code is giving you close to what you want:
$MailBody = $MailBody . "line 67".((isset($_POST["interet"]))? ($_POST["interet"]):"")."";
but you want to replace the commas with line breaks, so change it to:
$MailBody = $MailBody . "line 67".((isset($_POST["interet"]))? str_replace(", ", "<br />", $_POST["interet"]):"")."";