[Exim] Not immediately deleting delivered messages

Is there a _standard_ way to have exim keep all successfully delivered
mail in the queue for a fixed amount of time before actually deleting files?

I can think of several ways of hacking it in the config, if not.

The reason is very simple. One exim box does all the smap/virus filtering
as well as remote/specialist delivery. A subset of local accounts are
forwarded over to a MS-Exchange box for the users who like having their
mail over there. That box crashes occasionally, losing all mail since
the last backup of its database, and it would be nice to simply "undeliver"
the affected addresses (by modifying the journal files) so that the next
queue run will send the messages over to the other box again.