----- Original Message -----
From: "Benjamin Franz" <snowhare(_at_)nihongo(_dot_)org>
To: <spf-discuss(_at_)v2(_dot_)listbox(_dot_)com>
Sent: Sunday, January 09, 2005 12:14 PM
Subject: Re: [spf-discuss] Is anyone else getting DoS'd by relay attacks?
On Sun, 9 Jan 2005, Theo Schlossnagle wrote:
When a user is presented in the RCPT TO phase and that user does not
exist on the receiving system, then the mail server has two options:
(1) accept the mail _knowing_ that it will send an MDN later
(2) 550 5.0.1 user does not exist after RCPT TO:
(3) Accept it for delivery to the primary MX. Which _may_ trigger a MDN
later.
It is non-trivial in many cases for a secondary MX to know whether or not
the left side of a address is valid or not - except by attempting to
deliver it to the primary MX.
Or:
(4) Accept it for transfer to the internal MX/IMAP/POP server, which is
similar to (3) in its results but means that all email goes to an internal
user-based server from the external site-wide server. These setups are
surprisingly common for sites that have poor connectivity and numerous
external MX servers at scattered facilities.