we're planning move the actual e-mail service (post fix based) to Zimbra Open Edition.

One of our most concern is about the historical e-mails on each client user.

actually the users are configured as pop3 clients, and they have many historical e-mails.
We're thinking use the Zimbra Desktop as a new user client with IMAP configured. so, how to migrate the local historical's users e-mails from the old clients to ZD?

actually the users, use many version of clients (outlook express are most used, MS outlook, windows mail client).