[Exim4U] exim as a backup MX

Hi list, I have two exim4u installation but the mail boxes are only one server, this is done for backup purposes but I advertise both mx on the internet now the secondary server is accepting mails for my domains but can deliver them to any mailboxes as they don't exist on the second box. So I question is can I turn the second exim4u server to a backup MX? So that it can accept mails for my domains and queue/forward them to the primary(main) mail server. If so can you please send me the sample config file? Thanks Kebba