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Reverse DNS FAILED! This is a problem

We have just recently installed a new leased line for our internet services, which involved a new public IP address. Since then we are finding that some external recipients are not receiving email. We have a single Microsoft SBS2011 server running exchange.

On checking with MXTOOLBOX, we get the following error :-

Reverse DNS FAILED! This is a problem

I am sure that this is the problem, but how do I fix it??

We did create a new MX record on our ISP / Domain name host pointing to the new IP address, and for most parts it all seems to work, but I could do with getting rid of this error.

Does anyone have any ideas on how we resolve this issue?

Many thanks.

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8/22/2022 - Mon
Alan Hardisty

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Ian Pattison

You need to ask your ISP (who controls the IP addresses you've got) to add the reverse DNS pointer for your SBS2011 SMTP connector.

For example, if your SBS2011 is "remote.mydomain.co.uk" then that is what the Reverse DNS Pointer needs to have in it too.
Ian Pattison

LOL - was writing my response whilst Alan was too.

Basically we're saying the same thing - but I suggest you use "remote.mydomain.co.uk" which is what your server thinks it is!

we have an ahost record for remote.ourdomain.co.uk (for owa) and also one for mail.ourdomain.co.uk (for our MX record). Both point to the same destination, ie our public IP address.

I am not familiar with creating a revers DNS entry with our ISP. Is it something that they have to do, as I can only see the options for creating A and MX records.

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Walt Forbes
Alan Hardisty

It is something ONLY they can do.  You will need to call them.

Thanks to all

Some ISP control panels allow you to configure reverse DNS entries, but most require you to open a support ticket.

If they will not configure a reverse DNS entry, then its time to change ISP.

The reverse DNS entry allows you to look up the name from the address instead of the address from the name. This is used by most mail severs as a basic "reputation" check.

If however you use a third party "smarthost",  having a reverse DNS entry is not as important, but some mail servers will reject email if any mail server address in the chain does not have reverse DNS.
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