Speaking from personal experience dealing with this exact request quite often.
When a new employee replaces an old we use the "LDAP User Add/Update" and add the old users email to the new users permissions. This allows the user to view and search past emails sent/received from that terminated employee.
We do not delete the users from Active Directory, just move them into a disabled users OU. To make this work however we need to populate the "E-Mail:" attribute within Active Directory Users and Computers > General tab (for the disabled user) with their previous email. This may be unique to our environment however we have found that when you delete the users exchange mailbox the emails are not searchable without this small addition.
I would avoid PST files like the plague that they are.Go to the full post