I needed to give a user full control over every users mailbox in an Exchange 2010 environment. After getting the required permissions and authority, I used the following powershell command:
get-mailbox | add-mailboxpermission -user '%ALIAS%' - Accessrights 'FullAccess'
where %ALIAS% is the alias of the user who will be given permissions to the mailbox.
EG: Bob needs access to every mailbox:
get-mailbox | add-mailboxpermission -user 'BOB' - Accessrights 'FullAccess'