Problem using Zimbra web client inside an iframe - Internet Explorer

Hi,
I'm having a problem using the Zimbra web client interface inside an iframe on my page, on Internet Explorer (I am using IE9).
Unless I specify this meta tag on the main page: <meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7"> , it doesn't work.
What i get is the loading splash screen and the option to use the Standard html interface (which works fine, but is not what i'm looking for).

Any ideas on how I could get it working inside the iframe, but without specifying the meta tag ?

Thanks.