On 27 Feb 2019, at 12:36, Michael Peddemors wrote:
On 2019-02-27 9:27 a.m., Bill Cole wrote:
However, there are specific basic and enhanced SMTP reply codes which
are direct explicit statements that an address is non-existent which
should be honored immediately rather than taken as possibly mistaken
and retested later with a different message. For example, a 550 reply
to RCPT (or either stage of DATA, if there's only one accepted RCPT)
without any enhanced status code should ALWAYS be treated as an
indication of a non-existent address, as should 550 followed by any
5.1.x enhanced status code.
Hi Bill,
Do remember, in spite of RFC's there also is the perspective by many
SMTP implementations or operators, to NOT reveal whether an email
address actually exists, vs whether the message is non-deliverable.
Some implementations 'choose' to obfuscate the result to some extent
to help against dictionary attacks..
Which is a fine rationale for dropping addresses that chronically hard
fail in any way after a very small number of those hard failures which
may or may not indicate address non-existence (which I believe I did
advise.)
Something to consider..
Perhaps, but not really very relevant to what I said.
If the receiving system explicitly says that an address is non-existent,
a sender should believe them without re-testing. If they are explicitly
asserting address non-existence when the real cause of rejection is the
sender or the content, they are still telling the sender to stop mailing
that address and that request should be honored.
If you act otherwise, you can (rightly) damage your own reputation.
For example, I have used dead addresses (which have been kicking back
'550 5.1.1' replies to RCPT for extended periods, sometimes forever) as
spammer canaries. The same unfamiliar sender hitting one twice in a
short enough time will not be sending anything to any address on the
same mail system for longer than the average ESP lifetime, absent manual
intervention. This has been an effective and low-risk tactic for years.
--
Bill Cole
[email protected] or [email protected]
(AKA @grumpybozo and many *@billmail.scconsult.com addresses)
Available For Hire: https://linkedin.com/in/billcole
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