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Postfix Domain based Relaying

Posted on 2006-06-13
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I have 1 postfix server that recives mail for "domain1.com"
I also have an Exchange server the recieves mail for "domain2.com"

The two of them are on the same private subnet.
Most users use the postfix server to send email

I would like the postfix server to relay any mail with a final destination of "domain2.com" to the exchange server instead of looking up an MX server, and sending the message over the Internet only to have it come back to the exchange server on the LAN.
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Question by:tuaris
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by:Cyclops3590
ID: 16906612
just so you know I haven't actually done this before, but this Q should help you out
http://www.experts-exchange.com/Networking/Email_Groupware/Q_21863974.html
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matheweis earned 100 total points
ID: 16916649
I have done this before with both postfix and sendmail.

There's a lot of ways to approach this, but it depends on your setup. I need a little more information to tell you the best approach for your situation.

Are both your postfix and Exchange server open to the internet? (Have public IP's)

Does the postfix server send mail directly, or relay it through your ISP's mail server?

My approach is to simply use an internal DNS server on the same machine as the postfix server. Since the machine is configured to use it's own DNS server, I add MX records for what in your case would be "domain2.com" that point to the Exchange internal IP.
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by:tuaris
ID: 16921988
Domain2.com doesn't belong to us, it is not in my control and thier are some host names that the postfix server need to resolve on it.
Postfix sends the mail directly

Both Postfix and Exchange are open to the Interent, but the exchange server is connected to a "mother site" via VPN.  All email for domain2.com goes to that main exchange server.  Any messages that belong to us are sent to our mailboxes on our local exchange server.

The postfix is already acting as a fowarding DNS server for benchmarkmanagement.com
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by:matheweis
ID: 16922549
Well, there are several approaches to solve your problem. Most of the ones I would use involve tricking the postfix server using DNS, and/or using aliases to forward e-mail to the Exchange server.

From your original post, it sounded like you wanted ALL e-mail for domain2.com redirected to a specific server (Namely, you Exchange server) If that's the case, what I mentioned before will work:

It doesn't matter whether you have control over domain2.com or not. You can still create a dummy domain on the postfix server with mx records pointing to your internal Exchange server. Of course it does involve setting up a DNS server, but it sounds like you have that done already. The question is whether or not this is the solution that best fits your needs. Do you know much else about domain2.com? Does all e-mail that belongs to domain2.com go to you, or is it delivered to others as well? How often, if ever, will it's IP's will change - if they do, will you know about it, etc. Judging from what you are saying, it sound like this may not be the best option.


Fom your previous message, it now sounds like you are asking to forward e-mail with a destination of domain2.com, but only specific users of domain2.com, to a specific server of domain2.com, but not the main server for domain2.com...

Alternatively, you can simply set up aliases in postfix to foward messages only for the users directly on YOUR Exchange server. This will do exactly what you want, unfortunately it's not the easiest thing to set up.

a. Choose some dummy domain - for example domain3.com or mydummydomain.com. What the domain is really doesn't matter. You don't need to register it, because you're never going to use it outside of your local network.
b. In the Exchange server, you need to go to the "Active Directory Users and Groups" tool. In the properties of each user, under the e-mail addresses section, you need to add a new SMTP address (user@mydummydomain.com)
c. Create a DNS record for mydummydomain.com on the postfix server, with an MX entry pointing to the Exchange server.
d. Create a virtual alias table in postfix (More info on the here: http://www.postfix.org/VIRTUAL_README.html), with an entry for each user@domain2.com pointing to user@mydummydomain.com
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by:tuaris
ID: 16922764
Thank you for your help, I'll do the MX record tip.  I do want all the mail for domain2.com sent to the exchange server, that server will then do what it needs to do (keep the message or send it off to the mother server).

I just wanted to know if there was any other way to do this in Postfix besides using DNS.
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by:matheweis
ID: 16922977
If you want to do it with just postfix other than DNS, and you are okay with forwarding ALL mail from domain2.com to the internal Exchange server, using postfix transports:

In your postfix main.cf, make sure you have the following:
relay_domains = domain2.com
transport_maps = hash:/etc/postfix/transport

In /etc/postfix/transport, put
domain2.com     smtp:[10.0.4.89]

Where 10.0.4.89 is the IP of your Exchange server.

I haven't always had the best of luck with postfix transports, but this one is pretty clear cut, as long as Exchange is okay with the relay method.
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