I have the following code:

 
<a href="mailto:[email protected]">
   <label value="Request support"/>
</a>

Clicking the link opens the mail client as expected but as soon as you try to do anything else on the page, zk redirects automatically to the session timeout page.

Tested on ZKFiddle and it has the same behavior.

Check zk forum topic: http://forum.zkoss.org/question/94852/href-mailto-causes-session-timeout/#94862