Best Practices

Would like to know how people have handled the issue of terminated Help Desk / KACE Service desk staff, specifically as regards users responding to closed tickets owned by the terminated / disabled employee.  We thought we were all good for having removed their access and taken them out of the service desk labels, but recently had an issue that escalated to management when a user was responding to a recurring issue that was going to a former employee's email.  Since we remove term'd employees from active directory and exchange, we didn't know the employee was getting pinged.   What is the best way to deal with this?  Do you have to set up a rule or is there a forwarding feature I am overlooking?   Or do you just handle it on the Exchange / Outlook side of the email equation ?

Thanks for any help you can offer.