Allowing users to specify a forwarding address is a feature . Users can specify addresses to forward mail and calendar invitations they receive. You can check Don't keep a local copy of messages, and a copy of the forwarded message is not kept in the user's mailbox.
You can add forwarding addresses in the account's Forwarding tab and users can manage the forwarding addresses from their Zimbra Web Client Preferences Mail tab.
As messages comes into an account with forwarding enabled and an address configured, a copy is sent to the forwarding address.
In an account's Forwarding tab, you can specify forwarding addresses that is hidden from the user. When mail is delivered, a copy is forwarded to the address listed. The user does not know that the email was forwarded. If Don't keep a local copy of messages is checked, the message is not saved in the user's account.
When mail forwarding is disabled, the option to create a forwarding address and to disable local delivery does not appear on the user's Preferences Mail tab and Mail Filters tab.
If you disable forwarding but a forwarding address is already specified in the user's account, forwarding will continue. In this case, you must manually remove the forwarding address from the account's Mail preferences page.