Email Best Practice for Employee Separations: 

1. Block the employee from accessing company email on the server but allow the supervisor to respond to their emails.  Change their email password on the server. Create an email forward from the old employee account to their supervisor.  Add an auto-responder to the email address so senders  consult your company website for updated contact information.  Be vague in auto response messages so not to too easily broadcast the new email address to old spammers.
2. Move their computer to IT support.  IT support can spot check to verify old emails are fully downloaded. Do this by taking the computer off line and spot checking that imap email still opens with the full text and attachments.
3. Create the local email archive in an Outlook pst folder.  How do I backup my IMAP e-mail to my local drive in Outlook?
4. Verify the local archive and copy the resultant pst file to a network folder large enough to hold the archives.
5. Rename the archive with the employees name.
6. Burn a DVD with the employee’s email archive.
7. Delete the employee email account from the server and from their work computer.
8. Re-Create a forward from the old employee account to their supervisor and re-add an auto responder to the email address.
9. When the new employee is hired consider adding their predecessors email archive to their computer’s Outlook.  Consult legal counsel before sharing an ex-employees email archive with anybody.

Some of the steps above can be selectively implemented in advance for employees who’s email may contain vital operational data and/or when there are early discussions of separating employment.  In extreme cases IT can technically run Outlook IMAP on an an extra company computers to regularly archive all staff email.