about MTA's 4xx response code

classic Classic list List threaded Threaded
9 messages Options
Reply | Threaded
Open this post in threaded view
|

about MTA's 4xx response code

Eliza
I know postfix returns 4xx Response Code for Temporarily Deferred, as below:

----- The following addresses had fatal errors -----
[Status: Error, Address: <[hidden email]>, ResponseCode 421, 4.7.0
Temporarily Deferred]

Message will be retried for 4 more day(s)


I don't like every MTA returns this 4xx code, that would make the
incoming messages delay a lot time.

How do you think of this? should RFC reconsider to disable 4xx code?

regards.
Reply | Threaded
Open this post in threaded view
|

Re: about MTA's 4xx response code

Andrew Bernard

Hi,

Here's an extract from a site about 421:


SMTP Error 421

421 Temporary System Problem. Try again later.
421 Try again later, closing connection.
421 Server busy, try again later.

The SMTP error 421 is normally used for temporary problems on the mail server or a problem with the recipients email account.
Some mail providers might also return 421 after you reached a limit (restriction) on your mail account (see SMTP Error 451 below).


Does this help?

Suggesting the RFC be modified is off the table, because that is not the problem. RFC's are very seriously thought out and hugely important documents, tested and approved by serious committees. Looking. Sure, they get updated, but not for reasons related to issues like this.


Andrew



On 29/8/19 12:36 pm, Eliza wrote:
I know postfix returns 4xx Response Code for Temporarily Deferred, as below:

----- The following addresses had fatal errors -----
[Status: Error, Address: [hidden email], ResponseCode 421, 4.7.0 Temporarily Deferred]

Message will be retried for 4 more day(s)


I don't like every MTA returns this 4xx code, that would make the incoming messages delay a lot time.

How do you think of this? should RFC reconsider to disable 4xx code?

Reply | Threaded
Open this post in threaded view
|

Re: about MTA's 4xx response code

Eliza
Hello,

on 2019/8/29 10:42, Andrew Bernard wrote:
> 421 Temporary System Problem. Try again later.
> 421 Try again later, closing connection.
> 421 Server busy, try again later.
>
> The SMTP error 421 is normally used for temporary problems on the mail
> server or a problem with the recipients email account.
> Some mail providers might also return 421 after you reached a limit
> (restriction) on your mail account (see SMTP Error 451 below).
>

But I know some antispam systems get abused to return 4xx even there is
neither temporary system problem nor server busy. They just doubt this
is suspicious message and return 4xx codes.

regards.
Reply | Threaded
Open this post in threaded view
|

Re: about MTA's 4xx response code

wa6vvv
> On 28 August 2019, at 19:50, Eliza <[hidden email]> wrote:
>
> Hello,
>
> on 2019/8/29 10:42, Andrew Bernard wrote:
>> 421 Temporary System Problem. Try again later.
>> 421 Try again later, closing connection.
>> 421 Server busy, try again later.
>> The SMTP error 421 is normally used for temporary problems on the mail server or a problem with the recipients email account.
>> Some mail providers might also return 421 after you reached a limit (restriction) on your mail account (see SMTP Error 451 below).
>
> But I know some antispam systems get abused to return 4xx even there is neither temporary system problem nor server busy. They just doubt this is suspicious message and return 4xx codes.

Greylisting used to be a very effective approach to spam blocking.  When I first implemented it a number of years ago, it blocked about 95% of the incoming mail.  I believe the number of false positives was under 10.  However, greylisting was based on the high cost of queueing (disk space and internet bandwidth).  When the cost of those fell, spammers were able to implement queueing at very little cost.  As a result greylisting became fairly ineffective.  It was blocking less than 5% of the spam when I discontinued it.

I suspect that most of us who implemented greylisting considered the receipt of spam to be a system problem and hence the 400 series codes were appropriate.  YMMV.

-- Doug
Reply | Threaded
Open this post in threaded view
|

Re: about MTA's 4xx response code

Bill Cole-3
In reply to this post by Eliza
On 28 Aug 2019, at 22:50, Eliza wrote:

> Hello,
>
> on 2019/8/29 10:42, Andrew Bernard wrote:
>> 421 Temporary System Problem. Try again later.
>> 421 Try again later, closing connection.
>> 421 Server busy, try again later.
>>
>> The SMTP error 421 is normally used for temporary problems on the
>> mail server or a problem with the recipients email account.
>> Some mail providers might also return 421 after you reached a limit
>> (restriction) on your mail account (see SMTP Error 451 below).
>>
>
> But I know some antispam systems get abused to return 4xx even there
> is neither temporary system problem nor server busy. They just doubt
> this is suspicious message and return 4xx codes.

Which is their absolute right to do for whatever reason they see fit. It
is NOT "abuse" in any sense.

Andrew didn't mention the enhanced status codes of RFC3463, which
include the '4.7.0' you are seeing in error messages. That is a generic,
intentionally vague, and perfectly valid status code.

--
Bill Cole
[hidden email] or [hidden email]
(AKA @grumpybozo and many *@billmail.scconsult.com addresses)
Reply | Threaded
Open this post in threaded view
|

Re: about MTA's 4xx response code

Eliza


on 2019/8/29 11:30, Bill Cole wrote:
> Which is their absolute right to do for whatever reason they see fit. It
> is NOT "abuse" in any sense.
>
> Andrew didn't mention the enhanced status codes of RFC3463, which
> include the '4.7.0' you are seeing in error messages. That is a generic,
> intentionally vague, and perfectly valid status code.

Thinking the IM (like whatsapp), user send a message, the peer always
says: system temporary error, please try again. That would be a terrible
user experience. So as the email system. :)

Just my thought.
regards.
Reply | Threaded
Open this post in threaded view
|

Re: about MTA's 4xx response code

@lbutlr
In reply to this post by wa6vvv
On 28 Aug 2019, at 21:06, Doug Hardie <[hidden email]> wrote:
> Greylisting used to be a very effective approach to spam blocking.

Only if you were very careful monitor your system for the many many corporate sanders where greylist was the effective equivalent of a blacklist (those that did not retry in violation of RFCs and this that had many servers and would resend from different ones).

I tried it, but had far too many banks (refuse to resend at all) and mailers like google (many servers, never the same one twice within the greylist window) that I fairly quickly decided it wasn’t tenable.



--
Because you can't cotton to evil. No Sir. You have to smack evil on the
nose with the rolled-up newspaper of justice and say, 'Bad evil. Bad BAD
evil”'

Reply | Threaded
Open this post in threaded view
|

Re: about MTA's 4xx response code

Wesley Peng


on 2019/8/29 16:50, @lbutlr wrote:
> Only if you were very careful monitor your system for the many many corporate sanders where greylist was the effective equivalent of a blacklist (those that did not retry in violation of RFCs and this that had many servers and would resend from different ones).
>
> I tried it, but had far too many banks (refuse to resend at all) and mailers like google (many servers, never the same one twice within the greylist window) that I fairly quickly decided it wasn’t tenable.

Some open lists refuse to resend then those messages are discarded.
I once had a mail.ru account which have greylist policy thus open
mailing lists refuse to talk to it.

regards.
wesley
Reply | Threaded
Open this post in threaded view
|

Re: about MTA's 4xx response code

Matus UHLAR - fantomas
In reply to this post by Eliza
On 29.08.19 10:36, Eliza wrote:
>I know postfix returns 4xx Response Code for Temporarily Deferred, as below:
>
>----- The following addresses had fatal errors -----
>[Status: Error, Address: <[hidden email]>, ResponseCode 421,
>4.7.0 Temporarily Deferred]
>
>Message will be retried for 4 more day(s)

this happens when there is some temporary reason why the mail can't be
accepted/delivered.
Exampes are system maintenance, disk full etc

>I don't like every MTA returns this 4xx code, that would make the
>incoming messages delay a lot time.
>
>How do you think of this? should RFC reconsider to disable 4xx code?

do you want everyone to drop or reject mail just because of some temporary
problems?

--
Matus UHLAR - fantomas, [hidden email] ; http://www.fantomas.sk/
Warning: I wish NOT to receive e-mail advertising to this address.
Varovanie: na tuto adresu chcem NEDOSTAVAT akukolvek reklamnu postu.
Quantum mechanics: The dreams stuff is made of.