Additional details:
<iq id="wQghX-12" to="chris@54.201.91.132/54.201.91.132" from="pubsub.54.201.91.132" type="error">
<pubsub xmlns="http://jabber.org/protocol/pubsub">
<publish xmlns="http://jabber.org/protocol/pubsub">
<item>xxx</item>
</publish>
</pubsub>
<error code="400" type="MODIFY">
<bad-request xmlns="urn:ietf:params:xml:ns:xmpp-stanzas"/>
<item-forbidden xmlns="http://jabber.org/protocol/pubsub#errors"/>
</error>
</iq>