I'm afraid there's not much you can do about this one.
With one machine, no. These kinds of high-volume, widely sourced attacks, and spam in general reaching 70+% of all SMTP traffic, expose the irresolvable weakness of trying to defend against high-volume attacks with the mailbox server (and any add-in software that requires complete reception of every single message in order to reject) as the MX and as only line of defense.
But with a separate machine as MX front-end, the bulk of these messages, coming from other mail servers bouncing to MXs, would be blocked by string matching on the subject: header, mime headers, and/or body. The mailbox server, and most of all, the users would not feel the effects of the attack. The mail admin staff could also relax and have fun watching the carnage.
Len
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