Where Zimbra can't beat Exchange on is complete perfect integration with Outlook. It does however beat Exchange and Outlook on their offered functionality.
That's actually the key to the differentiation of Exchange. Integration with Outlook means integration with Office and the incredible morass of Office Automation that large businesses tend to build over time. Just as there was never a drop-in replacement for Notes, despite the detractors (usually Exchange fanboys) listing similar bullet-point by bullet-point comparisons and declaring victory, it's the ability for the groupware platforms like Notes and now Exchange to pull together a wide array of communication and messaging activities through desktop application integration that really gives them the leg up and lock in when it comes to the Enterprise.
Zimbra is, for sure, an outstanding messaging and communication solution that when you break down the bullets on the standalone Outlook/Exchange combo fares extremely well. Ain't always that simple. :)