I have a Windows 2008 server that is running an SMTP server. The server has a public IP Address. I have 2 other windows servers, not in the same network, that are used for hosting client applications. We have a need to make sure that the emails sent through the client application to the SMTP server look like they originate from the original domains. Right now, everything is coming from our mail server domain. So the traffic flow would look like this:
A request is made to www.test.com on port 25 to send mail. www.test.com routes the request to our SMTP server. The SMTP server (mail1.smtpserver.com) gets the request and sends out the email. The headers for the email states it came from www.test.com rather than mail1.smtpserver.com
I have heard either IP Tunneling or a VPN could help with this. I am not a network engineer so this is all new to me. I need to know what the best and easiest way is to set this up. I am familiar with DNS configurations. I have worked a bit with static routing, as well and read some posts about adding routes to the table. We just need to make sure the traffic is translated when leaving the SMTP server.