Incoming SMTP in the year 2017 and absence of DKIM

Owen DeLong owen at
Thu Nov 30 18:01:32 UTC 2017

> On Nov 30, 2017, at 09:55 , Bjørn Mork <bjorn at> wrote:
> Steve Atkins <steve at> writes:
>>> On Nov 30, 2017, at 1:22 AM, Bjørn Mork <bjorn at> wrote:
>>> "John Levine" <johnl at> writes:
>>>> Broken rDNS is just broken, since there's approximately no reason ever
>>>> to send from a host that doesn't know its own name.
>>> rDNS is not a host attribute, and will therefore tell you exactly
>>> nothing about the host.
>> It tells you something about the competence of the operator and
>> whether the host is intended by the owners to send email.
> No.  It only tells you something about the administrative split between
> IP address management and host management.
> There is no way my laptop is going to be able to update the rDNS for all
> addresses it will use in different networks.  This does in no way affect
> its MTA configuration.

Perhaps a better way to word it is “It tells us something about whether the
machine is likely to possess properties which make it generally undesirable
for us to accept messages from it directly.”

I, for one, have no interest in accepting messages into my mail server directly
from your laptop, even if they are legitimately from you to me. I’m perfectly
happy to insist that you go via an MTA hosted in a more permanent location on
your side first in order to avoid receiving messages directly from the much
larger quantity of incompetently administered mailservers, many of which I suspect
are not intended by their owners (distinct from their pwn3rs) to be mail servers
at all.

>> Or, for a more empirical way to look at it, there's reasonable correlation
>> between having missing, generic or incorrect reverse DNS and the host
>> being a source of unwanted or malicious email.
> Really?  Where did you get those numbers?  This is a myth.  Spam sources
> are average Internet hosts.  The split between working and non-working
> rDNS is mostly between IPv4 and IPv6, not between ham and spam.  And if
> there is some correlation there, then I'd say that an IPv4 host is more
> likely to be a spam source than a dual stack or IPv6 only host.

Really? Most of my hosts have working rDNS for both v4 and v6.

As to an IPv4 host being a more likely source of SPAM, I’m not convinced about
that, either given the amount of SPAM that hits my mailserver via IPv6.


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