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 IP lookup. If PTR record lookup fails (no PTR record) or PTR record is not forward confirmed or looks like generic, the message may be marked as spam or rejected.
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.
Many mail servers check PTR record against set of regular expressions to see if it matches a generic PTR such as "1-2-3-4-dynamic-ip.example.com" or "11-22-33-44-host.hostingcompany.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 generic PTR records or dynamic IP addresses.
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