LOST OF CONTRACT - HELP PLEASE

I HAVE THIS CLIENT WHICH WE HAVE SENT A CONTRACT LAST OCTOBER 2010 THRU COURIER IN THE U.S. AND IT WAS CONFIRMED THAT HE HAD RECEIVED THE SAID DOCUMENT.

WHEN WE EMAILED OUR CLIENT SOMETIME IN NOVEMBER IN THAT SAME YEAR TO FOLLOW WHEN IS HE GOING TO SEND BACK THE CONTRACT WE WERE ADVISED THAT IT WAS ALREADY MAILED BACK THRU USPS(POSTAL MAIL). UNFORTUNATELY UP TO DATE WE HAVE NOT RECEIVE ANYTHING.

UPON INFORMING MY CLIENT ABOUT OUR DILEMMA AND ASKED FOR AN AFFIDAVIT OF LOST THEY HAVE REPLIED AND SAID THAT THEY WILL NOT EXECUTE SUCH AFFIDAVIT.

THEY ARE ACTUALLY BLAMING US FOR NOT INFORMING THEM IMMEDIATELY WITH THE STATUS OF THEIR CONTRACT.

WE ALREADY MADE THOROUGH INVESTIGATION ABOUT THE INCIDENT, TRIED TO CHECK OUR RECORDS IF WE HAVE REALLY RECEIVE SUCH BUT THE RESULT IS NEGATIVE.

THE QUESTION IS, ARE WE TO BLAME FOR THE LOST OF CONTRACT., WHICH WE BELIEVE WAS LOST IN MAIL? CAN YOU GIVE ME SOME LEGAL ADVISE UNTO THIS ISSUE... WHAT AM I GOING TO DO???

HOPING SOMEONE WOULD ATTEND TO MY CONCERN...
AND LOOKING FORWARD FOR IMMEDIATE RESPONSE...
THANK YOU VERY MUCH....