why IPv6 isn't ready for prime time, SMTP edition

Rich Kulawiec rsk at gsp.org
Tue Mar 25 22:38:58 UTC 2014

On Tue, Mar 25, 2014 at 02:57:15PM -0600, Brielle Bruns wrote:
> Nothing wrong with my mail server setup, except the lack of RDNS.
> Lacking reverse should be one of many things to consider with
> rejecting e-mails, but should not be the only condition.

Lack of rDNS means either (a) there is something temporarily wrong with
rDNS/DNS or (b) it's a spam source or (c) someone doesn't know how to set
up rDNS/DNS for a mail server.  Over the past decade, (b) has been the
answer to about five or six 9's (depending on how I crunch the numbers),
so deferring on that alone is not only sensible, but quite clearly a
best practice.  If it turns out that it looks like (b) but is actually
(a), then as long as the DNS issue clears up before SMTP retries stop,
mail is merely delayed, not rejected.  And although *sometimes* it's
(c), why would I want to accept mail from a server run by people who
don't grasp basic email server operation best practices?   (Doubly so
since long experience strongly suggests people that botch this will very
likely botch other things as well, some of which can result in negative
outcomes *for me* if I accomodate them.)

Of all the things that we need to do in order to make our mail servers
play nice with the rest of the world, DNS/rDNS (and HELO) are among
the simplest and easiest.


p.s. I also reject on mismatched and generic rDNS.  Real mail servers have
real names, so if [generic] you insist on making yours look like a bot,
I'll believe you and treat it like one.

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