When an employee leaves a company, is it legal to have that employees email address forwarded to another company email or at the very least having it autorespond to the emailer that the employee is no longer with the company and please contact ........ whoever now with any questions?

When an employee leaves a company, the company may, but is not required to forward the employee's email to them. In most cases, however, I would advise against it. You create problems for your business if there is ever a litigation and you have been forwarding the employee his or her email. If you send a relevant email to a former employee outside of your company, you risk inadvertently violating your duties of document retention. For that reason I strongly encourage you to forward that employee's email to a responsible manager who can make a determination whether to forward his or her email to the employee on a case-by case basis. After all, the employee should have immediately let friends, family and colleagues know that he was no longer with your company, so there should not be many personal emails to sift through.