You may sometimes need to give yourself or another user Full Access permissions in Exchange on multiple mailboxes to search the mailboxes or to access messages in them.
This example returns all mailboxes from an Exchange server and then assigns the user ‘MyAdmin’ (replace this with the user you want to give permissions to) Full Access permissions on the mailboxes. If you only need to do this to a certain set of mailboxes you can change the Get-Mailbox command to use filters to target a certain range.
Get-Mailbox -ResultSize unlimited | Add-MailboxPermission -User MyAdmin -AccessRights FullAccess -InheritanceType all
Once done you can remove the Full Access permissions with the following command.
Get-Mailbox -ResultSize unlimited | Remove-MailboxPermission -User MyAdmin -AccessRights FullAccess -InheritanceType all