You can setup vacation responder in Gmail’s settings to reply anyone (as per your settings) who emails you automatically. Just go to Gmail’s Settings menu, scroll down and enable “Vacation Responder on” option.
Enter the subject and body of your message in the Subject, check the box next to “Only send a response to people in my Contacts” if you like so and click “Save Changes” option. To stop the auto respond feature, just enable “Vacation Responder off” and save the changes. That’s it.
