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On our other (non-Zimbra) Postfix boxes, we use a lookup table to reject emails where the MX record for the sender's domain points to an A record with a private IP address.
We have found this technique to be pretty effective, and it doesn't use a lot of resources either.
I filled out a Bug Report to request this enhancement be included in Zimbra. If you like it, please vote for it! It's bug # 11667
Bugzilla
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Source
Anti-Spam Postfix Enhancement Request 
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