A few quick guidelines:
- If you have more than 250 users it is cost-effective to use our full-scale migration technology. It's completely server-to-server, leaves end-users out of the loop, and has results as though you've been using Exchange calendaring all along.
- For a few hundred users or under, try our Decaf option. Application here. Documentation here. It's a server-side ICS import to hosted Exchange, and right now it's mainly geared towards Oracle Calendar migrations, but we'll open it up to Zimbra users if we see enough demand.