fraud e-mail with "to:" and "from:" the same

Customer reported us e-mails when "from" and "to" are the same (it is customer's e-mail) E-mail is not sent of course from our server. To: some.user@domain.com From: some.user@domain.com In postfix it was a simple option (reject_unauth_destination), to prevent such fraud e-mail. But exim in cPanel receives such e-mails and customers are confused. How to prevent it ? w.