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I've setup an SMTP server in our DMZ to send emails out via postfix for a website we host.

If I want to send email outside our network from that server but emails being received will be going to our Exchange server, does the server still need a public MX record?

I am thinking the answer may be yes to whitelist that server as an accepted sender of email from our domain but I'd like some clarification.

Thanks in advance.

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up vote 9 down vote accepted

No you don't need an MX record. An MX record designates where email for a domain goes, not where it comes from. Anyone using the MX record as a test to determine if the sending server is valid is doing it wrong.

What you should do is make sure you have an SPF record for your domain that designates this server as a valid sender for your domain.

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Okay, so I need an SPF record. Thanks...this is what I was looking for. – Windows Ninja Feb 23 '12 at 13:18
Glad to help. Make sure to accept my answer, if you in fact accept it. – joeqwerty Feb 23 '12 at 13:22
The OP doesn't need an SPF record if he's just sending to another Exchange server within the organization. The SPF record would be useful alongside a reverse PTR record if they intended to send from the Postfix server to the world. – ewwhite Feb 23 '12 at 13:27
The OP stated he's sending to the outside world from this server: "If I want to send email outside our network from that server..." – joeqwerty Feb 23 '12 at 13:29
PTR record is absolutely necessary. I don't tend to use SPF records, so that's not a must-have. You also have the option of relaying through your Exchange server (see "smarthost" in my answer). – ewwhite Feb 23 '12 at 15:38

No. You won't need an MX record to SEND... However, you may want to make sure that you have a reverse PTR record for that host if you're sending to the world. If you're only interested in sending to your Exchange server, you can either set the Exchange server up as a smarthost (relay) on the Postfix system or add a receive connector in the Exchange server with the Postfix server's details.

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