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550 Reverse DNS lookup failed

Posted on 2014-10-09
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Last Modified: 2014-10-27
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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Question by:Cobra25
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Expert Comment

by:Jan Springer
ID: 40371177
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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Author Comment

by:Cobra25
ID: 40371196
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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Expert Comment

by:Joshua Grantom
ID: 40371201
I would update your DNS records for mail.yourdomain.com
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Author Comment

by:Cobra25
ID: 40371207
Joshua please be more specific we are not even using a DNS record named mail.
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Expert Comment

by:Joshua Grantom
ID: 40371220
you didnt provide your external facing hostname for your mail server so I used mail.yourdomain.com as an example.
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Expert Comment

by:Jan Springer
ID: 40371237
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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Author Comment

by:Cobra25
ID: 40371252
They match by name, but different IPs
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Accepted Solution

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Joshua Grantom earned 500 total points
ID: 40371263
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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Expert Comment

by:Joshua Grantom
ID: 40371269
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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Author Comment

by:Cobra25
ID: 40371374
Should i change the NAT so the mail server goes out as the .66 address?
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LVL 16

Expert Comment

by:Joshua Grantom
ID: 40371385
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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Expert Comment

by:gheist
ID: 40371695
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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Expert Comment

by:Gareth Gudger
ID: 40372197
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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