Login to the webmail facility at CheaperDomains
Click on Settings in the top right.
Select Filters, which is the last option in the first column on the left.
Select "roundcube" in the middle column with the title "Filter sets".
Click "+" (a plus sign) under "Filters".
Specify "Filter name".
Select "all messages" For incoming mail.
Choose an action "Reply with message".
Fill the other fields with the required information (auto-reply message).
When finished click save.