[Exim] separate archiving for incoming and outgoing?

I am a relatively new convert to exim from sendmail. I need to maintain a
running backup of mail sent and received by the server, and I'm using the
system filter suggested in the FAQ. My question -- is there a way to
backup mail received for local users separately from mail sent by local
users? I would like to have copies of incoming mail sent to a user called
"backup" and copies of outgoing mail sent by local users to a user called
"sentmail".

Can anyone suggest a way of adding this kind of distinguishing capability
to my filter?

Here's the filter:

# Exim filter

# Ignore error messages
if error_message then finish endif


# Copy if this is the first delivery attempt
if first_delivery then
unseen deliver backup@??? errors_to postmaster@???
endif


--
Mark Edwards
San Francisco, CA