Confirmation for fake mail

my self a seller & other side is a purchaser the purchaser issued a chaeque against my balance payment the cheque was dishonored. then i filed a complaint against him u/s 138. In evidence he has produced a print copy of Email which print is fake (formatted in word Documents) how can i prove it. i have not send that mail to him he is showing mail was received in gmail box but i have sent him only two mails in yahoo mail box