If you're not looking to include all Mailboxes from a particular O365 Organisation, you can follow the steps below.

  1. Go to [Mailbox Backups]

  2. Next, simply find the particular O365 Organisation you'd like to exclude mailboxes from and click on the spanner icon.

    At this point you will get 2 options;

    [Disable Backups for this Mailbox]
    Disabling Backups for this Mailbox will stop backups from taking place in the future. The Backup Data will remain available and can be restored from the Restore Wizards. You will still be charged a monthly fee for this Mailbox. This feature is useful to retain backups of a Mailbox that has been deleted or archived in Office 365. To re-enable backups in the future simply choose to 'Reenable Backups for this Mailbox'.

    [Remove Mailbox from Subscription & Delete Backup Data]
    Removing this Mailbox will stop backup and restore operations from taking place and delete all Backup Data. From next Month you will no longer be billed for this Mailbox. If you would like to Backup this Mailbox again in the future you can include it in your Subscription but older backup versions will not be available.