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

Mark