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The Reverse DNS PTR-records (or Pointer Records, or Reverse Record) are often incorrect. Like pointing to:

  1. the domain of a hosting provider;
  2. a non-existing domain.

Let's say I have example.com hosted on IP 10.13.3.7. But the PTR record of 10.13.3.7 points to stackexchange.com.

Now, when I send email from 10.13.3.7 using @example.com email addresses, a "better" SPAM filter will notice that the PTR record points to stackexchange.com instead of example.com and give my email a higher SPAM score.

But, does the fact that a PTR-record is incorrect anyhow introduce another security risk for example.com or stackexchange.com?

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There's rather a lot of preconceptions here.

the domain of a hosting provider

...does not mean that the PTR record is wrong. An IP address can have multiple names associated with it (either directly in A records or indirectly via CNAME records) but a PTR can only have one name.

a "better" SPAM filter will notice

No, a better Spam filter will check the SPF record to see if 10.13.3.7 is a legitimate sender for example.com, and if it proves not to be, then give your email a higher Spam score.

A lot of the added processing implemented around UBE is not based on agreed standards (SPF is a notable exception) hence a lot of solutions are rather badly designed/implemented. A lot of them also rely on not disclosing the criteria by which they differentiate between good and bad requests.

Hence a PTR record which doesn't close the loop with the A record should not have an adverse impact, it may do in practice.

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