[Pkg-exim4-users] Reverse DNS lookups

[Florian Weimer]

* Bill Horne:

> 1. Sender is not in a dynamic address pool, but doesn't have a PTR
> record assigned.
> 2. Sender has an "A" record from a secondary DNS service, which points
> to the address in question.
> 3. I'd like to accept emails which have an "A" record that points to
> the IP address the email is sent from.
>
> Please tell me if this is a good idea, and how to do it if so. All
> suggestions welcome.

I use HELO verification for a similar purpose.  With the correct HELO
argument, a client can provide a better host name than which is
available from DNS (automatically generated PTR record, or nothing at
all).

As Andreas wrote, a true reverse lookup (from IP addresses to A
records) is not possible using standard DNS, but partial solutions
exist.