Message Rules.
Create a text file with the message you want to use as the reply.
Set up the rule:
For all messages
Reply with message
When you click on "message", change the file type to "Text (*.txt)
Navigate to the text file you created and select it.
Now, select "Stop processsing more rules"
The reason to use a text file is so that the text will go into the message
body unline the .eml file will be sent as an attachment.