[exim] email archiving to files using router and transport?

Hi,

I manage a number of domains who want to dump all email sent to/from
their domains to a file before delivery.

I'm wondering how I setup a router to match every email that's allowed
to be sent or accepted and parse it to a transport which then saves the
message to file and possibly tags the message (eg X-archive: this message
has been archived by the ARCHIVE-BLAH service) then continues onto the
rest of Exim's routers for normal processing.

Ideally, email for each domain would be stored automatically like this:
/var/log/exim/domainA.com_mail_received.log
/var/log/exim/domainA.com_mail_sent.log
/var/log/exim/domainB.com_mail_received.log
/var/log/exim/domainB.com_mail_sent.log

I'm new to Exim - can anyone help me out with the syntax?
Thanks, Andrew.