Quoting Paul Howarth <paul(_at_)city-fan(_dot_)org>:
First off, hello Mr. MCIVTA. ;-)
The reference implementation of SPF (the perl Mail::SPF::Query module) has a
mode of operation where mail from hosts that are listed as MXes for a
recipient's domain is allowed through (on the basis that SPF checking will
have been done on that MX).
The other implementations will no doubt have similar functionality because
obviously it's a big problem otherwise.
But that's still a problem. You've removed the admin overhead, but I now have a
trust relationship implicit in the MX records for the domain. I suppose
ultimately this means that I end up trusting the backup MX in the same way I do
for any host sending mail for a non-SPF protected domain. I'm not entirely sure
how I feel about this, but to me it looks like a hole to get through SPF that
spammers are going to target, especially when there are backup MX services out
there handling mail for tens of thousands of domains.
--
Paul Robinson
http://www.iconoplex.co.uk/
"God doesn't play dice." - Einstein