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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 on port 25 to send mail. routes the request to our SMTP server. The SMTP server ( gets the request and sends out the email. The headers for the email states it came from rather than

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.

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Are your '2 other windows servers' in a datacenter, or on a home internet connection? It matters... trust me – Mike Pennington Jul 30 '12 at 21:35
@MikePennington, they are located in a data center. They are not dedicated, but VPS, by the way. – Robert Class Jul 30 '12 at 21:38
up vote 1 down vote accepted

No need for IPSEC Tunnel or VPN, just use SMTP relay with SSL and auth. Change the relay servers SMTP properties to use a TLS certificate, allow relay for authenticated users (keep checkbox to require TLS) and have them connect that way. You can have them use port 25 or the standard "secure SMTP" port of 465 by adding that to the SMTP listener.

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The servers are sending mail directly through my server now how would setting up a relay help? The other servers are not running SMTP servers for me to setup the relays from => to. Also, even if I had a relay, wouldn't the headers of the email still look like they originated form the SMTP server? – Robert Class Jul 31 '12 at 11:30
I apologize, I just tested a simple relay on my server and it does seem to do what you suggested. Will I need to run SMTP servers on my other machines to relay the messages from those servers to my SMTP server? – Robert Class Jul 31 '12 at 12:37
Your main Server 2008 is acting at the SMTP relay in this scenario. A relay is just a SMTP server that re-sends emails elsewhere that's not destined for it. You have not discussed what is running on the other two servers so I can't help you there. No you don't have to run the Windows SMTP service on the other two, but that is an option. – Bret Fisher Jul 31 '12 at 17:26
the other servers are just running IIS7 and serve up web applications. So, let's say Server A is my SMTP server and Server B is an application server (ASP.Net MVC3 apps). Server B has a domain, pointed to it on IP An application sends a mail to That traffic needs to be routed to Server A to be sent. I am not sure how to setup a relay without having 2 SMTP servers involved. – Robert Class Jul 31 '12 at 17:41
Your .NET app has two choices: 1. you code it to point directly to Server A, specifying hostname, port, username/pw, etc. in code. 2. You install SMTP service on each .NET server, tell .NET app to point to localhost for submitting, and then change the SMTP config in MMC console to relay all it's email through a "smart host" which is Server A. Number 2 is quite common. – Bret Fisher Jul 31 '12 at 22:19

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