The information from your host indicates they recently made some changes to the availability of wget on the server.
If
$http_program = "/usr/bin/wget43 -O /dev/null " . $listmail_url . "/bounce.php";
doesn't work, maybe:
$http_program = "wget43 -O /dev/null " . $listmail_url . "/bounce.php";
omitting the direct path, will.
Another alternative that might work is curl
$http_program = "curl -o /dev/null " . $listmail_url . "/bounce.php";
Regards