The expected behavior is for the email to be forwarded and it will either be marked as read or deleted from the inbox (depending on POP3/IMAP and configuration). So it should not be repeatedly processed after it's forwarded. If you find that this is happening, you can enter a request on the My Support Portal for us to look into it.
Foward unprocesses messages comes into picture when incoming messages cannot be processed correctly by service email. This can be because of various reasons like unexpected message formats, listener processing configuration.
As of today, all the unprocessed messages are to be handled manually. Hence, we need to specify Forwarding email address for unprocessed emails so that the owner of that email can look at the email messages and take manual action.
This is because of the fact that once an email in the inbox is read by email listener, the email will be updated in the inbox as read, so email listener cannot do anymore attempts of processing as it is configured to process only the unread emails.
May be the admin of the email box can mark the email as unread manually and then see if the email listener picks up it again.