[ALUG] ISP-relay SMTP question

[(Ted Harding)]

Thanks, John. Some comments interspersed below.

On 15-Nov-07 13:46:14, Jon Dye wrote:
> On 15/11/2007, Ted Harding <ted.harding at nessie.mcc.ac.uk> wrote:
>> My question is: What (if any) is the relationship between
>>
>>   <whatever.my.sendmail.uses>
>>
>> and
>>
>>   <whatever.my.ISP.uses>
>>
>> ??
>>
>> Does <whatever.my.ISP.uses> take the email address from
>> the "From:" header in the email? From the "envelope-from"?
>> Or does it uses its own version, as in (e.g.):
> 
> It should use the envelope-from.

That could be useful! How definitive is that?

>> One aspect of the background to this question is that,
>> when I try myself to do a (test) manual SMTP dialogue with
>> one of the remote domains mail servers, while I may well
>> get through and successfully send the mail, on many other
>> occasions I will get a response to "mail from:" on the
>> lines of
>>
>>   mail from: Ted.Harding at manchester.ac.uk
>>   451 4.3.2 Please try again later
>>
>> which will be repeated indefinitely if I keep trying.
>> So I'm wondering if it may be a negative response to the
>> email address used in the "mail from:" request, or has
>> some other reason.
> 
> When receiving the temporary rejection you are supposed to retry
> later.  This is normally because of something like too much load on
> the server or a (temporarily) full disk.

I understand that. However, at times I've retried every 5 minutes
or so for up to 10 times, always with the same result. (I use
manual telnet to port 25 on the remote server, then doing the
EHLO ... / mail from: ... / rcpt to: ... [if I get that far] / ...
manually).

> It could also be that the mail server may have greylisting of some
> sort.  Basically this is a scheme where the first time you connect to
> send email the server tells you to try again later.  When you retry
> (with the same from and from the same server) it lets the mail
> through.  The thinking is that spam sending mail bots won't bother
> with the retry and will just fail.  The problem is that some
> legitimate servers do the same.

Yes,I know about grey-listing (which is probably in place at the
far end). It seems to me, though, that the above failure of re-tries
possibly rules out grey-listing, since it should respond positively
to the next try or two.

>> [And, by the way, how should I interpret the error message
>>   451 4.3.2 Please try again later
>> ??]
> 
> It basically means that the server can't accept email now but
> that it should be able to at some point so you should try sending
> it again later (e.g. re-queue it for delivery).

Yes, 451 is supposed to indicate

  451 Requested action aborted: error in processing

If I'm not mistaken (though I could be ... ) the "4.3.2" is
an instance of the RFC 3463 status nessages class:

  X.3.2   System not accepting network messages

  The host on which the mailbox is resident is not accepting
  messages.  Examples of such conditions include an immanent
  shutdown, excessive load, or system maintenance.  This is
  useful for both permanent and persistent transient errors.

which, I guess, could also be valid for a grey-listing refusal.

Hmmm ...
Ted.

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Date: 15-Nov-07                                       Time: 14:17:26
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