Google has added a new feature to Gmail, enabling you to let people know you won't be able to get back to them right away.
You can set up a vacation response in your Gmail account that will automatically reply to anyone who emails you. While the vacation responder is enabled, Gmail will send a response to anyone who contacts you.* If that person contacts you again after four days and your vacation responder is still enabled, Gmail will send another vacation response to remind the person that you're away from your email.
More info about this new feature can be found over here.