Incoming SMTP in the year 2017 and absence of DKIM

Keith Medcalf kmedcalf at
Thu Nov 30 20:47:56 UTC 2017

On Thursday, 30 November, 2017 10:55, 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:

>> 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.

Your Laptop should not be an MTA.  Perhaps it is a authenticated submission agent sending to MTA, but without properly configured forward/reverse DNS it is not an MTA.  Many systems will not accept SMTP from it unless it can authenticate.

>> 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.  

You are incorrect.  If DNS is not configured correctly then the spam to ham ratio is pretty much 100% spam with no ham.

>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.

Actually, you are incorrect again.  In order of "Spaminess" (most spammy first) you have the following order:

IPv4 with incorrectly configured DNS.
IPv6 without regard for DNS configuration.
IPv4 with correctly configured DNS.

More information about the NANOG mailing list