Re: How to do a defer_if_permit with a SMTP code 451?

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How to do a defer_if_permit with a SMTP code 451?
\ Kim Minh Kaplan (28 May 2007)
. \ (Wietse Venema) (28 May 2007)
. . \ Kim Minh Kaplan (28 May 2007)
. . . \ (Wietse Venema) (28 May 2007)

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Subject:	Re: How to do a defer_if_permit with a SMTP code 451?
Group:	Postfix-users
From:	(Wietse Venema)
Date:	28 May 2007

Kim Minh Kaplan:
[ Charset ISO-8859-1 unsupported, converting... ]
> On 5/28/07, Wietse Venema <wietse> wrote:
> > Kim Minh Kaplan:
> > > Hello,
> > >
> > > I have written Greyfix? (yet another) greylisting policy server for
> > > Postfix. The best current practice in Postfix to defer delivery for
> > > greylisting seems to be to return action=defer_if_permit. The problem
> > > is that this rejects the mail with an SMTP code 450 whereas it would
> > > be better to use an SMTP code 451?.
> >
> > What evidence do you have that this will change client behavior?
>
> Evidence is too strong a word. But some experiments I did about a
> month ago with Gmail seem to indicate that they use a very large pool
> of servers (more than /24 network) and that after a 450 response they
> would change mail server across their /24. Now that I changed to a
> 451 response they retry using close mail servers. I'll make these
> experiments another time to verify if they changed their system during
> this one month period. If you are interested in the resulting logs, I
> can post them here.
>
> Please note that the initial idea that it will change client behavior
> is not mine. I stole it from
> http://cvs.puremagic.com/viewcvs/greylisting/schema/whitelist_ip.txt?rev=HEAD
> which is the home of the Relaydelay greylisting software :
>
> please check that your implementation uses the 451 error code (not 450
> or another 4xx code). Some problems have been reported for sites like
> MSN/Hotmail, Prodigy, and various other senders that appear to be
> having "weird" retry patterns (sometimes resulting in bounces) when
> using code 450 or others.

I would like to see some evidence. There is a lot of information
on the internet that gets copied from one site to the other without
proper verification.

Not that I would be surprised that there are broken mail systems,
but if they can't properly retry after 4xx then they will likely
have lots of problems.

Wietse