How Zimbra will set up the mailboxes?

I have ZCS version 6.0.13 that runs on Ubuntu 8.4 that I would like to set up to validate against a list of Active Directory users on another Windows server. I have read the Zimbra wiki and understand how to configure this, but I’m worried about how Zimbra will set up the mailboxes. Most of my users’ mailboxes are now in Zimbra, and I want existing users to keep their original mails. Will their mailboxes be affected if I setup an external authentication with Active Directory?
