Steps to create auto reply mail on plesk:

• Log on to plesk 
• Goto mail area
• click email address auto reply option 
• click on the auto reply checkbox and add the following settings:

1. Auto reply Subject message
2. Message text format: its recommend you use plain text formart, because some user might not be able to view formatted text
3. Auto reply message to send to all recipents
4. Forward To: if you wish to forward recieve msg to another mail address, click to add the desired mail you wish to forward to
5. Attach file: click this button to select a file to send with auto reply message to recipents
6. Switch off auto reply: select this button if you want to disable auto reply
7. click ok when done to apply the settings.

Read How to create email accounts in Plesk Panel