Pretty sure that the zimbra admin interface doesn't expose this sort of option. But also reasonably sure that amavisd can do this (which Zimbra uses)... look at the /opt/zimbra/conf/amavisd.conf and amavisd.conf.in
The amavisd.conf contains the settings that zimbra uses, this conf is generated from amavisd.conf.in (everytime you start zimbra, I think) - a few warnings, it is likely that you can mess things up, so tread carefully, and any changes you make will be lost upon upgrades, so you'll want to track them. Good luck!
PS - I'm not a zimbra guru, so some of what I said above could be incorrect. But, I also have used amavisd-new before on a gateway, and tho' I have not done what you are looking for, I've found amavisd to be pretty flexible. So there is your 'caveat emptor'
