[exim] Migration from Exchange to Linux

I have a client that had a Linux mail server, but a person who was not
Linux-friendly replaced it with a server running a trial version of MS
Exchange. The client only needs a simple mail server and doesn't want
to pay the license fee to keep using Exchange, so I am exploring whether
they can move back to Linux without losing all stored e-mail.

I can't find any information on converting the aggregate and proprietary
MSExchange data file to the Linux maildir files I know and prefer.

Does anyone have any ideas or techniques for doing this? Perhaps a way
to script the sending of e-mails from the Exchange server to an Exim server?

Thanks.

-- Angelo