A search for "outlook pop do not delete messages from server" comes up with some results for configuring the mail clients. There is a similar feature in thunderbird, believe in the accounts settings > server settings.
However, with 200 users, you probably do not want to rely on them to configure the application on their own, and letting users make configuration changes is usually not a good idea. I would try copying each message as it is received to another directory for backup purposes. Then a cron job to copy it to a remote location every night, deleting after a successful completion of the backup. That or switch to imap.
But then again, that is just my opinion