Instead of POP3, use IMAP. All your emails would be stored on the server. Make sure your server got enough space to store all the employees email.

By default, all emails is protected under the law. It should not be view or accessible by other people. Person who wanted to know about the email content then he/she should be in the CC or BCC list. That's the purpose of CC and BCC. Cannot simply open ex-employee PST file like that. Local companies still have the issue with this. MNC will straight delete the email account once the employee leaving the company premises on his/her last day of work.

IMAP still the best solution for them. You can set to have the alert user when reach 80% quota. Atleast when they go oversea for holiday for 1~2 weeks, the mailbox won't full. User have to manually clear and housekeeping their email.