Spammers have found that sending spam to both the primary MX and one or
multiple backup MXs often works great to increase the number of spam
messages being delivered. Often, backup MXs are less well configured to
stop SPAM.
Do you have any research results that could underpin this problem
statement?
I'm only one data point, but my experience is the exact opposite. One of
my honeypot systems has 5 MX records, the first four of which are
directed to Postini, and the fifth that goes straight to the mailbox.
I've been frankly surprised at the negligible amount of spam received by
that 5th MX host. Even a sustained directory harvest attack (concurrent
to an attack on my ssh server) last week didn't reach that 5th MX host.
<csg>
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