An automatic reply is a confirmation that the sender’s mail has been received by you. It is useful when you are not in a position to answer your emails (e.g. while on vacation).
Go to ‘Settings’ > 'Set Auto Reply'. Specify the subject in the 'Subject' field and type the contents of the email in the box below. Click on 'Activate my automatic reply' to activate the response mail and click 'Save'. Now whenever you receive a mail, the sender shall receive your message automatically.