Client hired our candidate but said never got resume

Client hired our candidate but said never got resume

I recently found out that one of our clients hired a candidate we sent through Zoho as "forward to client"  It shows under the company as well as the candidate that he was submitted.  How can I get proof that it was indeed sent and showing the attachment as well?

This could be a potentially huge problem if it shows that resume are being sent in Zoho but our clients are not receiving the emails.  Just this one deal is $30,000