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.
--
Benjamin Franz
"All right, where is the answer? The battle of wits has begun.
It ends when you click and we both serve pages - and find out who is right,
and who is slashdotted." - David Brandt