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Reverse DNS test

Computer networks use the DNS to determine the IP address associated with a domain name. This process is also known as forward DNS resolution. Reverse DNS lookup is the inverse process of this, the resolution of an IP address to its designated domain name.

While receiving an email message, a mail server may try to attempt reverse DNS lookup. If the lookup fails (no PTR record), the message may be marked as spam or rejected.

 

AOL reverse DNS requirements

To reduce spam rate AOL rejects email originated from servers that do not comply to requirements described here. To ensure your email delivery to AOL and many other recipients our proactive blacklist monitoring service performs FCrDNS test and dynamic IP address test.

 

Check reverse DNS

IP address or host name:    

 

FCrDNS test

FCrDNS, or Forward Confirmed reverse DNS, is when an IP address has forward and reverse DNS entries that match each other. For FCrDNS verification, first a reverse DNS lookup is done to get a list of PTR. Then for each domain name mentioned in the PTR records, a regular DNS lookup is done to see if any of the A records match the original IP address. If there is a forward DNS lookup that confirms one of the names given by the reverse DNS lookup, then the FCrDNS check passes.
Example:

IP address 1.2.3.4 resolves to mail.domain.com.
Host name mail.domain.com resolves to IP addresses 1.2.3.4 and 5.6.7.8.
Thus, reverse DNS for IP address 1.2.3.4 is forward confirmed.

 

Dynamic IP test

Many mail servers, including AOL, check domain names retrieved using reverse DNS lookup to see if they are likely from dialup users or dynamically assigned addresses. Owners of such IP addresses typically assign them generic names such as "1-2-3-4-dynamic-ip.example.com." Since the vast majority, but by no means all, of email that originates from such addresses is spam, many spam filters refuse email originated from such sources. Here is an example of typical bounce message

#5.5.0 smtp;550 We do not accept mail from dynamic IPs.

Debouncer tests if your reverse DNS record match widely used naming patterns used to detect dynamic IP addresses or generic PTR records.

 

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