Can you copy your schema extension here ? It's difficult to help without more elements. It can come from your extension, but to be sure we have so see it.
Also, when you go to the nms:recipient schema edition, even if you can't edit it, you can see the generated schema (with its extensions) on the second tab 'preview' on the bottom. I don't know if it can fit into a message, but it would be very usefull if you can copy it here.