James Wilkinson writes:
Guy Fraser wrote:...snip...Mail servers do not generally accept a DATA command if the RCPT command produces an error, so the rest of the headers are not looked at. The proper response is to respond with a user undeliverable error.That assumes that the receiving server knows that the address is unknown. Very often (as with westexe.demon.co.uk), the MX (server to which e-mails get sent initially) is owned by a big ISP (in my case Demon Internet), which doesn't know which addresses on westexe.demon.co.uk are valid. So it has to accept all e-mails to westexe.demon.co.uk.
Tough cookies.Poor network design/configuration is not a valid excuse for a denial-of-service attack.
I have several thousand IP addresses blacklisted, for trying to flood me with bounces to forged spam.
Any mail server bouncing forged crap to me, for any reason, gets blacklisted. End of story.
In this case, the relevant standards (RFC 821 and 2821) say that since the e-mail has been accepted but can't be delivered, a bounce message *must* be sent. (These days, there's enough spam and viruses about that this is no longer considered best practice.)
In general, it's not easy to program a MTA to be sufficiently sure that an e-mail *is* faked that it can drop it.
Actually it is. I've designed my mail server so that it should never generate backscatter. Even due to traditionally difficult to anticipate situations, such as a mailbox over quota.
It's not rocket science. It can be done. It has been done.
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