Log onto your plesk account.
On your Dashboard, click on 'Mail' on the left-hand side of the page.
From the list of email address on your account, click on the address of the email you would like to switch auto-reply on for.
Near the top of the page, click on 'Auto-Reply'.
This page will allow you to switch on auto-reply and type in the reply you want to send. You can also set a date for when you want the auto-reply to turn off.