[Exim] Archiving all email with Exim4

We're using Exim4 on a debian server with about 150 accounts. This is a
new setup for us. Because we are a govenment agency (sewer district) I
need to archive ALL emails, preferably to a single file that I can
backup periodically and clear or rotate. Is there a standard way this
is done with Exim? On my previous server, I was using Sendmail and had
a script that worked with procmail to scan and store a copy of every
email. I thought I'd ask for suggestions here before trying to recreate
my old system.

All suggestions welcome. Thanks!

Art Mandler
MIS Admin
Metropolitan Sewerage District of Buncombe County, NC
amandler@???