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Rackspace provides both Exchange and IMAP email accounts. Depending on which type of accounts you have, it may be required that you include user passwords in your migration configuration.
When migrating from Rackspace Exchange, it is recommended you create a support ticket with Rackspace requesting they configure the Impersonation role on your admin account. This will allow your admin account, with the Impersonation role applied, to authenticate all user accounts without including user passwords in your migration configuration.
When migrating Rackspace IMAP accounts, you will need to include user passwords in your migration configuration, as Rackspace does not support admin authentication with IMAP accounts.