I would like to suggest an improvement to zimbra.

There has been an answer to this before in the thread: "Forwarding only accounts" and it works:

set up the local account as a distribution list:
zmprov cdl list@domain.tld
and add the external account to the dl:
zmprov adlm list@domain.tld acct@external.tld

The thing is, we should have a checkbox in the forwarding config page that says "copy mails locally" to be able to clear the local zimbraMailDeliveryAddress instead of having to create a distribution list because for one, we cannot create mail aliases on distribution list and for two, it is ankward to create a distribution list since in a normal ISP setup, it is appening very often that mailboxes are external forward-only boxes.

just a suggestion