A new mailbox is one that is new to Enterprise Vault. When you first start using Enterprise Vault, all the mailboxes are new. With this setting selected, all existing mailboxes are enabled when the Domino Mailbox Archiving task runs. All mailboxes created in the future will also be enabled. The associated archives are automatically created.

If there is a mailbox that you do not want to enable, you can explicitly disable it by running the Disable Mailbox wizard. This prevents the mailbox being enabled automatically, so the mailbox is never archived unless you choose to enable it.