If you're dedicated to stopping IE users from accessing Zimbra, you can sit Zimbra behind an HTTP proxy (Apache2/nginx) and do some User Agent detecting. Granted it's not foolproof; but most IE users wouldn't know what a User Agent string is let alone how to change it.
For example, in Apache:
Code:
BrowserMatch MSIE iexplorer
Deny from env=iexplorer
And
using nginx.