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Email rejected because of rDNS

Posted on 2007-12-05
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Last Modified: 2008-02-01
Some of our emails are rejected with this message:

There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            <mail.example.org #5.5.0 smtp;554 <unknown[1xx.xxx.xx.x]>: Client host rejected: rDNS/DNS validation failed. Please setup matching DNS and rDNS records.

The email IP address resolves to mail.example.org and our main IP address resolves to host.example.org.

 It looks like the receiving side of the email sees the email coming from the host IP address instead of the mail IP address, so because it cannot solve the DNS it rejects my email.
How should I solve that? Why do they see it coming from the host IP address instead of the mail IP address.

Please let me know if you need more details

Thank you for your help!
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Question by:adrian1976
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by:bhnmi
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When you do a reverse lookup (PTR) on the IP that the MX record resolves too, does it match the host that is defined in the MX record? Sometimes is you are not the SOA for you block of IP's it will resolve to a record created by your ISP.
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by:adrian1976
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Thank you for your answer!

 Our MX records resolve to a third party spam filter solution. We forwarded our MX records to them, so all incoming emails are getting through their filters and then get to our email server. Our outgoing emails are not going through these filters. The IP address of the filter is different from the ones I mentioned in my first post. Could this setup be a problem?

Thanks!
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by:Chris Dent
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Hi,

Ignore the MX as it doesn't define the server which sends Outbound Mail.

For outbound mail your SMTP Server must have a valid Public Name, e.g. mail.yourdomain.com. That Name must exist in your Public DNS Zone and link to the Public IP of your Mail Server (or Firewall address if it just uses NAT to get there).

Then you must ask your ISP to add a Reverse Lookup Record (PTR) for the Public IP Address which, in turn, points back to the name (e.g. mail.yourdomain.com).

Chris
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by:adrian1976
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Chris-Dent,

Your suggestion is very helpful. Thanks!

As I specified in my first post, I have different public IP addresses for my firewall and for the email. would there be any problems if I were to change the firewall IP address public name from host.example.com to mail.example.com? Should I have both IP addresses with the same name (mail.example.com)

Thank you for your help!
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by:Chris Dent
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There's nothing to stop you doing that, no. There's nothing really to stop you adding that as another record as well if you choose to.

As long as when your server, when it connects to another mail server, has a valid name and a PTR record that takes it from the Public IP it connects with back to that name.

Chris
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by:adrian1976
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Forgive my ignorance but if both public Ip addresses would have the same name (mail.example.com), wouldn't that be a problem?

Thank you!
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by:Chris Dent
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Not really, the most important part here is to get the Reverse Lookup Record in, so the IP address the server connects with resolves back to a name.

The record you mention above is the Forward Lookup Zone, we just need the name we use there to resolve to that IP. It doesn't matter if another name also resolves to that same IP.

Hope that makes sense.

Chris

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by:adrian1976
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It makes sense. I will make a request to my ISP to change it.

Thank you for all your help!
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Chris Dent earned 400 total points
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You're welcome.

Chris
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