Andriy G. Tereshchenko wrote:
As well PTR look up require at least additional 2 DNS requests, while
exists will require only one.
Most mail servers anyway do a PTR lookup on the connecting IP address, and
the good ones confirm it by checking if the A records points back to the IP
address. So the 'ptr' mechanism requires zero or one additional DNS
requests.
Now look at the email policy of hotmail.com: it does not fit into one
record. You need two to four DNS requests to read it. By using 'ptr'
mechanisms you could reduced it to one record. That means less DNS requests
when using 'ptr'.
I see clear benefit from "exists".
In most cases 'exists' lookups are not cached whereas PTR lookups are
cached.
PTR is an evil
Then don't publish it.
Roger