Here's what will work.
As I understand it the problem is: when ppl leave before they resign, they delete all emails. Currently all emails are at your ISP.
You want: a way to have a copy of all emails whether sent/received. when they leave, their delete is of no impact.
Here's a partial solution
I would not give them ISP email access.
I would setup a local zimbra that I let them use.
I then setup their email at the isp, say
user@domain.com
Zimbra runs on domain.org
At ISP I forward a copy of everything that comes in to their zimbra account,
user@domain.org
Now, when they leave, all they have access to is zimbra. If they delete all zimbra email so what. You can get it back via the ISP account. This solution however doesn't account for the Sent emails.
If you backup zimbra daily, which you should, if they delete emails, you can get it back. That get's you everything in their account.
Solved, not pretty but, solved
Good luck with this