Go to Mail > Preferences and select “Rules”.
Click on the “Add Rule” button.
In the “Description” text field, name your new rule.
In the Drop-down menus under “If any of the following conditions are met:”, choose “Account” and then the email account to which you would like the rule to apply.
In the Drop-down menu under “Perform the following actions:”, choose “Reply to Message” then click the Reply message text...” button to insert your message.
Insert your message into the “Reply Message” text box.
Click “OK” when finished.