Re: [exim] Blocking Users with No Reverse DNS

On Mon, 5 Nov 2007, Phil (Medway Hosting) wrote:

| I return a 450 for all mail with no rDNS. That then allows time for DNS to
| recover if there were a delay or fault.

The 4xx response seemed to be the advice in the past, in the hope that a
temporary DNS problem gets fixed. The downside is any legit sender with
no rDNS at all (broken or otherwise) has to wait days while their message
festers on their outbound queue, before they get informed you're not
accepting it.

Note that exim (since version 4.50) also has host_lookup_deferred, to make
it easier to distinguish a DNS tempfail (where you issue 4xx) from a DNS
hardfail (where you might well want to issue 5xx). See Michael H's post
for example usage of this.