If you are leaving CERN for a while, you can forward your mails to another email address. Then incoming emails to your CERN Mailbox will be redirected to the specified email, leaving a copy or not in your CERN mailbox.
If you are leaving CERN but you want to be listed in CERN Phone book, simply change your Visible Email address.
To configure your visible Email or a Forward, go to either:
MMM Site (http://cern.ch/mmm): Services → CERN Tools → Visible Email address and Forwarding.
Outlook Web Access: CERN Tools → Visible Email address and Forwarding.