Default papers

[Kelso22]

I have no idea where my estranged husband has been for quite a few years. I finally tracked his mother down and asked her. She did not know either. She contacted her son and he sent me an e-mail telling me to send the divorce papers to his mom's address, as that was his permanent address. His mother signed for them. My default papers were returned to me stating that I needed proof that they were delivered to the respondent. I have his mother's signature and his e-mail telling me to send them to her house. Is this enough proof?