[exim] how to backup messages during a short period ?

Hi,

I would like to have a backup of all messages going thru exim:

the mx only relay emails in one way , from outside to inside
(an other server relay emails for the outside)

is it possible to deliver mail and keep a copy in a spool backup
directory for a short period of time
and let exim remove the messages older than ie 24h ?

thank you for your help

Patrice