How do i grant Full Access permissions to all mailboxes for a user or a group on on Exchange 2007

You can run the following PowerShell command
get-mailbox -Server <Server> | Add-MailboxPermission -User ‘<Domain>\<Username or Group>’ -AccessRights ‘FullAccess’
Example:
get-mailbox -Server SERVER| Add-MailboxPermission -User ‘DOMAIN\Domain Admins’ -AccessRights ‘FullAccess’
You can also run this from a batch file with the following command:
PowerShell.exe -PSConsoleFile “C:\Program Files\Microsoft\Exchange Server\Bin\ExShell.psc1” -Command “get-mailbox -Server <Server> | Add-MailboxPermission -User ‘<Domain>\<Username or Group>’ -AccessRights ‘FullAccess’