Receipt required and Non-Repudiation of receipt when the original user message is signed

Hi Theo and all,

I have a question about receipt generation. 

First if the original User Message is digital signed, is the receipt required to be generated and sent back by the message receiver?

Secondly, if the receipt is to be generated, does the receipt have to be non-repudiation (signed) also in the case that the original User Message is signed? Or it can just be Reception Awareness?

The spec seems not mandate these but I would like to ask for confirmation.

Thanks,
Rong