Exchange or Office365 + Comarchive
http://www.comarchive.com/
All mails are tracked, and stored in archive.
Only Works with Exchange Office365
The copies are stored on a fileshare+sql data, so it takes up less than 15% of the Space it would on a exchangeserver
This way, when you on/offboard new people, you dont have to worry about making copies or exporting their inbox.
In the EU where I live, Places like government and bankingcompanies are requiered by law to have an setup like this to make sure evidence cant be "misplaced"
The users cant delete from the archive.. only the admin