Archive the mailboxes first by selecting, then from the action menu select 'Archive'.
Upload mailboxes via CSV File upload.
- Login to Power365
- Use the top left menu to navigate to Groups + Teams
- Click the Office 365 Groups and Teams tab
- Click the Update action above the view
- Click Download Example List
- Use this example file to build your mapping list
- After building your list, save as a CSV file
- Click Select File
- Locate the CSV file
- Click Next
- Review any errors, otherwise upload is complete
- Verify the changes in the management view