Anothers sharing is your achieving
When we migrate Mailboxes to Office 365, it’s best to use an Admin Account for accessing the mailboxes, instead of knowing every users password.
But we might not have a user with FullAccess to all mailboxes we want to migrate, not even the Domain Administrator.
Lets use PowerShell With Exchange Cmdlets to achieve FullAccess instead of changing each user manully.
First I collect all mailboxes in a placeholder:
$mail = Get-Mailbox
Then I execute FullAccess to a User for all Mailboxes in my placeholder:
$mail | Add-MailboxPermission -AccessRights FullAccess -User Administrator