On Wed, Nov 30, 2005 at 09:51:38AM -0500, Stuart D. Gathman wrote:
system might perform. Only if the receiver system accepts the DATA
command and the message data with a 2xx status code, should the
forwarding/relaying/proxying system "accept" the message vis-a-vis the
calling system. (Or of course if the receiver system isn't reachable.)
As a backup MX, you might consider accepting the message only when the
reciever
system *isn't* reachable, rejecting the rcpt envelope with 4xx when it *is*
reachable. Normal senders should use the primary MX when available.
Only spammers will try a backup MX first, hoping for weaker spam
defenses.
Unfortunately it is hard, if not impossible, to determine that the
sender cannot reach the primary MX. The best you can achieve
is to determine that at the moment you check, the primary MX can
reach the sender. In order to do this you need to verify the
connection between primary MX and the sender, not between backup
MX and primary MX.
Alex
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