Log in to SmarterMail interface for your domain name (http://mail.yourdomain.com) using your full email address and password.

Step 1: Click the 'Settings' icon in the left panel

Step 2: Expand the 'My Settings' node if it isn't already and click 'Auto-Responder'

Step 3: Check the 'Enable auto-responder' option

Step 4: Click the 'Auto Responder Message' tab

Step 5: Create a subject and body for your auto responder

Step 6: Click the 'Save' button near the top

To turn off the auto-responder when it's no longer needed, uncheck the 'Enable auto-responder' option under the 'Options' tab and click 'Save'.  

Check 'Limit responses to once daily' and click 'Save' to restrict the auto-responder message to once per day per email address.