ietf-smtp
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SMTP delivery strategies

2004-08-27 11:34:17

A while ago there was a long and emotional discussion on the qmail list
about whether a 5xx should be treated as permanent for the message or
permanent or one server that is in the MX list of the addressed domain.
This originates from the fact that some ISPs don't accept email from
dialup address space and signal a 5xx (permanent) to the sending MTA,
however they mean "we do not accept this message from that IP - try your
ISPs relay SMTP server".

I did some tests with spam honey pots (add backup MX records, because
spammers deliver often to backup MX hosts to circumvent policies at the
best MX and backup MX hosts have usually higher trust at the best MX) and
noticed a lot of different behaviour of various MTAs that tried to backoff
(like some do contact other MX hosts on a 4xx message, others don't).

RFC2821 is not too clear about these strategies and some of the
confusion derives from the fact that there are no clear definitions
e.g. what "delivery" means.

Would it be a good idea/is there any interest in having a kinda BCP
document to get these clarified?

        \Maex

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