On Fri, Feb 20, 2004 at 08:20:24AM -0500, Theo Schlossnagle wrote:
There is no good reason not to enforce _all_ of your policy rules on
your secondary MX.
There is - scalability and manageability.
Working at an ISP offering backup MX service to customers on leased lines,
running zillions of different MTAs on different platforms, there is no way I
want to get involved with replicating their policies.
There is a bigger reason not to, though: because backup MX servers are
rarely used, any discrepancy in policy will lead to very occasional mail
problems which are extremely hard to debug.
If you have full control over both boxes, then of course you're free to
implement the system you describe. But I think for many people the benefit
is unlikely to outweigh the costs.
Remember that almost all Internet mail has already been through one relay:
namely the smarthost at the sender's ISP. So giving a 550 response to a mail
with a forged sender doesn't help anyone much, because the smarthost is then
responsible for delivering the bounce, which it cannot.
Implementing SPF on all smarthosts will improve the quality of sender
addresses. But either way it doesn't matter that the mail has been through a
second relay (the backup MX) before the destination address is fully
validated.
Regards,
Brian.