How to hide the client IP sender and show only the SMTP server IP

sendmailsmtp

We got a backend server in a specific ISP with a reverse proxy frontend in a different ISP. In order to send mail we use the backend server which connects to a frontend sendmail SMTP server. The problem is that we do not want the backend server to be public for the e-mail receiver, like it currently happens: 

Received: from smtp.ourdomain.com (thebackendserver.com [BA.CKE.ND.IP])
    (authenticated bits=0)
    by smtp.ourdomain.com (8.13.8/8.13.8) with ESMTP id oBE2K1NT020323
    (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO)
    for <receiver@anydomain.com>; Tue, 14 Dec 2010 08:20:05 +0100

Any solution for this?

Best Answer

From RFC 2821, section 4.4:

SMTP servers MUST prepend Received lines to messages; they MUST NOT change the order of existing lines or insert Received lines in any other location.

So I'm not saying it's impossible to do what you want, but do be aware that you'll be violating the RFCs. Undesirable behaviour often results from that.

What do you fear will happen as a result of these (mandatory) lines being added by your interior mail servers?

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