Headers show BE LAN IP

[lebisol]

Hello people,
I just noticed that my headers are revealing private IP in headers. Isn't this more info than headers should show?

Microsoft Mail Internet Headers Version 2.0
Received: from mail.domain.com ([xxx.xxx.xxx.xxx]) by mail.recipientdomain.com with Microsoft SMTPSVC(6.0.3790.3959);
  Wed, 22 Aug 2007 10:05:00 -0700
Received: from backend.domain.com ([192.168.1.x]) by mail.domain.com with Microsoft SMTPSVC(6.0.3790.3959);
  Wed, 22 Aug 2007 10:04:56 -0700

xxx.xxx.xxx.xxx is my public IP pointed to FE (mail.domain.com) as registered.

Is there a way to hide back-end LAN IP info?
Thanks!

:--------------------------------------:

All around in my home town,
They tryin' to track me down...