Before you attempt to migrate from the Exchange server, create a communication plan to notify users about the new Zimbra email program and to provide users with the new mail access information (Zimbra URL, login name, and password to use).
Zimbra recommends that you plan to perform the migration during off-peak use times. The Exchange server does not need to be stopped in order to migrate accounts. Any emails that are sent to or delivered from Exchange after the account is migrated are not saved in the Zimbra mailbox.