As the email is stored on the users workstation and not on a server you will need to get some sort of backup utility installed on all the users workstations which will allow you to backup the users machines, probably to a server in your office.
If that isn’t going to work you can look to getting an Exchange server for your office which will hold all the email on the server, then if the users workstation crashes all the email will still be saved. This is a pricy option as the license cost of Exchange server is fairly high.
You can also considder leaving a few days worth of email on the POP3 server so that if the workstation does crash they can recover at least a couple of days worth of email.
Improved by #### John Bostock #### Exchange Me!
If you wish to go down the road of workstaton backups:
Workstation Backups: See Here Desktop Version
Improved by #### Nick Mitchell (kb3cgj) ####
I had a similar issue and the road we decided to go down was to install a firewall with archive abilities. We selected a Fortinet Fortigate firewall. The firewall archives all inbound and outbound emails and website URL’s and Instant Messenger logs. They also offer a product called the Fortilog that gives you long-term storage of all of this information of up to a year or more depending on traffic loads. We now use the data for recovery needs and forensic evidence when needed.
Improved by Buddy Farr
another product you can look into is North Seas, it is a small switch that links in between your network and the exchange server and does the same as the Fortinet Fortigate, (though I am not sure about the instant messaging backup). It will auto backup all emails in and out of your exchange system even before they hit the inboxes. so someone cannot receive an email then delete it before backups run so there is no trace. It backs up the email before it hits the exchange server so they can’t delete all traces.