Unfortunately there is no way to redirect the emails received during the varying periods between your mail server going down and the activation of the Continuity Service for each of your customers domains. This is because the emails queued up for delivery to your mail server (which are held for approximately 36 hours), are held on a different server to that of the server on which the Web portal is hosted and to which the new emails are now being redirected. And there is no technically feasible way to transfer one to another.