If the system is only run periodically then it will seem draconian every time it is run. Any unread mail is put in the trashcan after 7 days. Any read mail is put in the trashcan after 14 days, unless it has been archived or saved. Any mail message in the trashcan is deleted after 7 days. If you want to be lenient, give users more days grace, and save mail to archive media.
Some users never read their mail, it's that simple.
Identify why they are not using mail. You must setup contact with the personnel department if you haven't already. There are other system admin functions you need to run too when a user leaves. If the user will never use mail, then remove them from the mailing system. New word processor documents can support built in programs, usually used benignly.