What you want to do has more to do really with Postfix than with Zimbra.
No Starch Press's The Book of Postfix is a good resource with a lot of info on regular expressions.
The Postfix website also has links to lots of documentation regarding what you want to do. There is also a Postfix mailing list where the Postfix developers hang out on a daily basis.
Hope that helps get you started...
All the best,
Mark
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___________________________________ L. Mark Stone, CIO  "Uptime. All the time."
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