Message sending failure and returning same to sender with error code 406

@robbiehanson 

we are sending message and the same message(MessageId is Same) is returning back to sender:

	<error code="406" type="cancel">
		<blocked
			xmlns="urn:xmpp:blocking:errors">
		</blocked>
		<not-acceptable
			xmlns="urn:ietf:params:xml:ns:xmpp-stanzas">
		</not-acceptable>
	</error>

Is this blocked by receiver or something else?
Could you help us?

Thanks
