What does the Warning ‘Reverse DNS FAILED!’ Mean? – SMTP Diagnostic Tool

We wanted to give a bit more insight into the Warning from our SMTP Diagnostic tool about ‘Reverse DNS Failing’.

When a sending server makes a connection to the recipient server, the recipient server notes the sending IP address and performs a reverse lookup. This is done by sending a DNS query which returns a Fully Qualified Domain Name ( FQDN) registered for that IP address. If the sending SMTP address matches the domain, then it’s much more likely that the message is legitimate and therefore will be passed on to the recipient. If the IP address doesn’t match, it’s much more likely that the sending address was spoofed and therefore much more likely that it’s unwanted and could be considered spam.

A FQDN is associated to an IP with a valid PTR record. You want the domain name portion of the FQDN to match the domain of your email address. (e.g. if your sending addresses follow the convention of name@mydomain.com, your PTR record should contain something like mailserver.mydomain.com). Only the organiztion which controls and owns the IP can set a PTR record. PTR record queries are sent to the owner of the IP address which is the ISP, unlike other DNS queries which are sent to the DNS server of whoever owns the domain. For this reason Setting a PTR record on your own DNS servers is almost useless since no one is asking your servers.

To make any changes to your rDNS, you will need to contact your ISP or if you host your own DNS (rare) you will adjust it yourself. You will not be able to do this in your DNS control panel unless your ISP also hosts your DNS and gives you the functionality to add your own rDNS records.

4 thoughts on “What does the Warning ‘Reverse DNS FAILED!’ Mean? – SMTP Diagnostic Tool

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  3. Michael Horowitz

    Its not clear how this applies to shared email hosting offered by an ISP. The reverse DNS will always be that of the ISP and never that of any ISP clients. Thus, how can this work at all for companies that send email through the mail servers offered by an ISP?

  4. wendy@mxtoolbox.com


    We are a bit confused as our SMTP Diagnostic tool is designed for Inbound testing. Outbound IPs do not typically have SMTP Banners since they are only accepting Outbound connections, not Inbound Connections.

    To check these settings, we connect to the server and the server replies with an SMTP Banner. We take the IP address we connected to and we do a Reverse Lookup on the IP of the server we are testing, to see what the domain is. Best practices would recommend that your SMTP Banner includes the domain that is correlated to the IP.

    Thank you,


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