Usermin - Notify Sender

Notifying Senders You No Longer Use an Account:

If you wish to use a vacation message to notify senders that you no longer will be using your account, you must delete any Forwarding Rule you might have set up, and only utilize the vacation message feature.

First, under Mail Forwarding, click on the link in the Forward to... section.
In the Edit window that then opens, hit the Delete button.

Enable your notification message under the Vacation Auto-Reply feature. (If set up correctly, you would then see the screen below if you checked back under the Mail Forward feature, indicating that only the vacation auto-reply rule is enabled.)