We've recently added an Exim-based (MailCleaner) MTA in our DMZ that sends and receives email to and from our LAN email server. It works great, but I'm a little wary of one of the headers it places in outgoing messages sent to external recipients.
Specifically, its the 'Received' header for the delivery from our LAN email server to the MTA in the DMZ, and it looks like:
Received: from [192.168.XX.XX] (helo=mailserver.localdomain.local) by mail.senderdomain.com stage1 with esmtp with id SomeMessageID for <firstname.lastname@example.org> from sendername <email@example.com>; Tue, 24 Nov 2009 13:06:58
Where 192.168.XX.XX is the DMZ interface of the LAN mail server, localdomain.local is our LAN domain name, and senderdomain.com is the externally-resolvable domain name for our organization.
Is it possible to modify this header so it doesn't divulge our local domain name and DMZ address range on every outgoing message? I assume we can't simply remove it, since out of the several 'Received' headers in the delivered messages I've been able to examine it's the only line that contains the 'from Sendername ' portion identifying the sender's email address in our organization.
Any hints about how to modify or mask this would be appreciated.