Creating mailbox (with forwarding / alias option) gives error through XML API

<?xml version="1.0" encoding="UTF-8"?>
<packet>
   <mail>
      <create>
         <filter>
            <site-id>12</site-id>
            <mailname>
               <name>testing</name>
               <mailbox>
                  <enabled>true</enabled>
               </mailbox>
               <password>
                  <value>testing123</value>
               </password>
               <forwarding>...