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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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.


smtp.example.com       =>  (A record)                 =>      smtp.example.com (will be configured as a PTR record)
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
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.
Joshua GrantomSenior Systems AdministratorCommented:
you didnt provide your external facing hostname for your mail server so I used mail.yourdomain.com as an example.
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.
Cobra25Author Commented:
They match by name, but different IPs
Joshua GrantomSenior Systems AdministratorCommented:

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.
Cobra25Author Commented:
Should i change the NAT so the mail server goes out as the .66 address?
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....
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.
Gareth GudgerSolution ArchitectCommented:
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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