Hi S.Mishra,

Thanks for posting here. I understand that you are doing a staged Exchange Migration, and I would like to confirm the following additional information:
 1.Did you create a .csv similar to feed to the migration tool?
 2.Did you assign the super-user or administrator account permissions to access mailboxes in your Exchange organization?

I think the root cause should be lack of permission for the super user. Exchange Control Panel-based migrations of on-premises mailbox data require that an account that has full access to the on-premises environment be used when the migration is performed. Use the following resources to confirm that your on-premises environment is configured correctly. Be aware that the migration service account will require the Receive As permission to the mailbox database or the Full Access permission to the individual mailbox that has to be migrated.
For Exchange Server 2007:
Use the Add-MailboxPermission cmdlet to add permissions to a mailbox
How to allow mailbox access

For Exchange Server 2003:
Microsoft Knowledge Base article 268754 - How to assign users or groups full access to other user mailboxes
Microsoft Knowledge Base article 821897 - How to assign service account access to all mailboxes in Exchange Server 2003

After that, please re-run the staged migration.

Additional information:
=====================================================
Plan for User Identity in a Staged Exchange Migration:
http://technet.microsoft.com/en-us/exchangelabshelp/hh312791
Migrate a Subset of Mailboxes to the Cloud with a Staged Exchange Migration:
http://technet.microsoft.com/en-us/exchangelabshelp/ff959224

Thanks,
Allen Qiu