How to backup e-mail of outlook express

In our company, colleagues like using Outlook Express for sending and receiving mails. We use IMail Server 8.0 as mail server program.

For safety, colleagues need to backup their mails periodically. Normally they just upload the Outlook Express folder (for example C:\Documents and Settings\hunterzh\Local Settings\Application Data\Identities\{94AE18B9-D135-480C-8957-CB859126F3D7}\Microsoft\Outlook Express) into our internal ftp server.

But how can, let?s say, they store their previous mails (before this January) in a folder, and continue to use other features like account, address book to send and receive new mails?

If just copy the Outlook Express entire folder, it?s hard to look back an old mail or divide them by time sections.

How can we satisfy the above requirements?