On Jan 22, 7:40pm, John Levine wrote:
}
} setup sends an auto-ack and puts the mail into a holding pen. If the
} auto-ack bounces, he moves the message into the spam folder.
This sounds very similar to the "callback" mechanism employed by (for
example) Verizon. During the SMTP exchange, between getting MAIL FROM:
and issuing a response, Verizon's servers attemp to connect to an MX
at the sending domain and issue a RCPT TO: that address. If Verizon
can't connect or gets a failed response, it issues a 450 response to
the original MAIL FROM:.
This is pretty effective from Verizon's viewpoint, but bombards any
forged domains with HELO/MAIL FROM:<>/RCPT TO:<...>/QUIT, which some
of those domains interpret as an attack.
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