Questions

Automated reply ("I am Out to Lunch" or anything you like ) is one of the many mail features you have in your site's Control Panel ( cPanel ).
To set this up in your cPanel go to Auto-Responders and add one - there is a helpful video guide.
Once you have added an Auto-responder it will show in your list of Current Autoresponders - check that address is correct.

Notes:
-- the mail box ( email address ) for the responder must already exists - do not set up onholidays@mysite.com if you have no such email address !
-- check the Autoreponder is working by sending email to its address - modify again, if necessary

Instructions on accessing your site's control panel (cPanel) are on: ›Namecell.net/hosting/ (type your Domain name in the box).