XML-RPC API Problem setting email forwarding addresses

I am receiving "error 1014: Element 'forwarding' is not expected." As far as I can tell I'm following the spec exactly. Any ideas why this breaks?

<packet version="1.6.0.2">
<mail>
<update>
<set>
<filter>
<domain_id>1</domain_id>
<mailname>
<name>test.user</name>
<forwarding>
<address>test1@domain.com</address>
<address>test2@domain.com</address>...

XML-RPC API Problem setting email forwarding addresses