Please note that to follow the below procedure to enable the Auto-Reply:
1.Login to the plesk control panel.
2.Click on the customer.
3.Click on the domain.
4.Click on the "Open with control panel"
5.Click on the mail tab.
6.Click on that mail id
7.Click on Auto-Reply.
8.Enter your massages in the auto-reply.
9.Click on ok.