Log in to your control panel.
Select the domain name that you want to apply the change to.
Click on MAIL.
Select the mailname that you want to modify.
Check the "Mail autoresponders" box.
Click on ADD.
Enter a name for the autoresponder.
Configure the properties for the autoresponder.
Click on UPDATE to take you back to the email configuration page.
Click on UPDATE.

PLEASE NOTE: Autoresponders normally sent out a reply each time an email is sent to them. If you would like to configure the autoresponder to behave like a vacation reply, you'll need to set the "Reply to unique e-mail address" to one or more days. This is important, because of you have a vacation reply send out a response for every email, you will probably be removed from any mailing lists that this address is subscribed to.