550 Reverse DNS lookup failed

Hey guys,

We cutover to a new ISP and we use Exchange 2010 w/SBS 2011. Now i did setup the rDNS with the ISP and it checks out fine on mxtoolbox and were not on any RBL's. We get a few messages daily bounced back due to #550 Reverse DNS lookup failed for our IP address.

What could be causing this?
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Cobra25Asked:
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Jan SpringerCommented:
Your forward fully qualified domain name needs to match its inverse.

Ask your provider to set up a PTR record for the IP of your outbound mail machine.

i.e.,

smtp.example.com       =>     192.168.4.21  (A record)

192.168.4.21                 =>      smtp.example.com (will be configured as a PTR record)
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Cobra25Author Commented:
So i double checked, this is how it's setup

server 10.10.10. 5 ---> x.x.x.70 (has rDNS with FQDN)

Now when you do the forward lookup on the same FQDN its actually pointed to x.x.x.66
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Joshua GrantomSenior Systems AdministratorCommented:
I would update your DNS records for mail.yourdomain.com
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Cobra25Author Commented:
Joshua please be more specific we are not even using a DNS record named mail.
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Joshua GrantomSenior Systems AdministratorCommented:
you didnt provide your external facing hostname for your mail server so I used mail.yourdomain.com as an example.
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Jan SpringerCommented:
Your forward and inverse DNS has to match.

So, if the forward is correct, fix the inverse.  If the inverse is correct, fix the forward.
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Cobra25Author Commented:
They match by name, but different IPs
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Joshua GrantomSenior Systems AdministratorCommented:
Cobra,

Your external dns is hosted somewhere, that record is pointing to the wrong IP so you need to change it
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Joshua GrantomSenior Systems AdministratorCommented:
do you not know where your domain dns is? Godaddy? Network Solutions? Maybe you have it on an external facing DNS server?

We cannot help any further without the domain name.
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Cobra25Author Commented:
Should i change the NAT so the mail server goes out as the .66 address?
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Joshua GrantomSenior Systems AdministratorCommented:
If you do that, then you will have to change the rDNS record also.

Instead, you can change the ip that your external host name is pointing to....
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gheistCommented:
Also DNS failures are cached in DNS servers for some time. typically 5 minutes to hour with config limit of about week. So just wait a little and problem will go away.
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Gareth GudgerCommented:
Should i change the NAT so the mail server goes out as the .66 address?

I would definitely make sure your mail server is always sending out on the same IP. Otherwise you will need to configure rDNS for your entire IP block.
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