[exim] Backup for Exim emails when using POP

Hi,

Can anyone suggest a way to perform a centralised backup when client use
POP protocol access Exim email server?
As far as I know, we may be able to setup Exim to make a copy of every
incoming/outgoing email to a single email account (eg. backup@???).
I don't know how this can be done, but it can be done in Postfix and
Sendmail.

The second problem is when this single email account (backup@???)
holds all emails, if there is client lost all his emails in his destop,
how can I restore all his emails from the backup@??? account and
let him read all the backup emails as if he never logon before. Assuming
client use POP.

PS. I m using Exim 4.42 in FreeBSD 5.3.

Thanks
Sam