The Reverse DNS PTR-records (or Pointer Records, or Reverse Record) are often incorrect. Like pointing to:
- the domain of a hosting provider;
- 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?