[Exim] Refilter messages?

Hi all,

I'm still searching the documentation for the ability to refilter
messages after they have already been received (i.e. they are in
/var/mail/$user).

I'm running Debian unstable and I'm using a .forward file in the
relevant user's directory. After I edit the .forward file I hope it can
be re-executed on the existing mail.

Regards,
Adam