can someone tell me why our Serendipity formats external links with the weird exit.php url?

<a href="http://www.altosresearch.com/blog/exit.php?url_id=1457&entry_id=245" onmouseover="window.status='http://www.reagentinct.com/2007/06/18/special-reader-feature-what-i-look-for-in-real-estate ...