What is a redirect or forward mailbox

A redirect or forward mailbox is an address where all your incoming emails are sent as a copy to.

Suppose you manage a mailbox with email address "example@domain.be" and you set the redirect as "example@gmail.com".
Any mail sent to example@domain.be is then forwarded or redirected to example@gmail.com.

So emails are always delivered to both places and are handled independently. If you delete the mail in your original mailbox, then it still persists in all your redirect mailboxes.