My suggestion is you ask that they set up an automatic reply in their outlook with the information a couple of weeks before they leave. I would guess these are for employees who left on good terms. Do you really want to be responsible for what happens to their email after they leave? That would be an administrative nightmare in my opinion. How long should you keep it up for them? Who is going to be responsible to turn it off after a period of time? Do you want someone wasting their time with this task for employees who are gone? Would they then hold you responsible if they miss an important email? Not a very good practice, to be honest.
Another possibility would be a simple out-of-office message, explaining that the person is gone and redirecting them to the appropriate person. You could even forward any incoming mail to that designate to avoid missing potentially vital business communications. One workplace had a Terminated group in AD, where disabled accounts were kept for a period established by Management (according to the likelihood of crucial messages continuing to flow) so that the email account would still function until it was determined that all contacts had been properly redirected. Then the account would be deleted from the system altogether.