[exim] Spambox per user solution?

Hi folks,

For some time I've been running Debian mail servers with Exim4, using
SpamAssassin and a bunch of custom ACLs to filter out the spam.

Messages seen as spam, such as with a high spamd score, used to be
denied outright. Later, I softened my policy and changed it to
"defer", which often gives me the opportunity to modify the whitelist
before a remote MTA ceases to make further delivery attempts. Luckily,
the necessary configuration changes for this were trivial.

Now, however, I'd like to go further and create a system that
maintains a spambox per user. Usually this system should accept the
messages it recognizes as spam, but then deposit them in the user's
spambox.

Can anyone recommend a simple or elegant way to achieve this?

Thanks,

Jaap