Migrating from Exchange
This is a step by step guide to migrating your domain's email from Exchange to Zimbra. It will take some side trips along the way -- my example domain, for instance, has Exchange 5.5, and yours will probably be newer, and is behind an older MailScanner box, which you might not be -- but hopefully the 5000 foot view information will be of help to you, and you can figure out the 500 foot stuff yourself when it differs.
This is a work in progress, as long as this notice remains, and that's why it's not yet linked from anywhere, including categories. Once I'm done with it, I'll link it in.