When users are composing in Internet Explorer 9 the window seems to "Freeze" if you will. You can't click on any of the buttons at the top namely Send or Cancel or click your cursor to a new location it the composition box. But if you minimize the window and bring it back up or do anything like click the start button where it causes the whole screen to refresh then it "unlocks" so to speak. It is like the whole window will not respond to mouse commands.

This could possible be an Internet Explorer issue and not a Zimbra issue but any help tracking this out would be appreciated.

It is doing it on a wide range of Windows 7 Pro systems. Even high spec systems.