Hallo,
Beachte mal das Ende des Attributs target. Was soll javascript:;('
<a class="eins"
href="http://www.xyz.de/abc/index.php"
target="Newsletter"z
onClick="javascript:;('http://www.xyz.de/abc/newsletter.php','Newsletter','width=600,height=400,directories=no,toolbar=no,location=no,menubar=no,scrollbars=yes,status=no,resizable=yes,dependent=no,fullscreen=no')">Newsletter</a>
Heiner