source - http://mail.google.com/support/bin/a...n&answer=25922
How do I set up an automatic vacation response?
Going on vacation? No access to the Internet? No problem! Use Gmail's vacation responder 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.
Here's how to let people know you can't respond right away:
While the vacation responder is enabled, you'll see a banner across the top of your Gmail account, displaying the subject of your vacation response. To stop Gmail from automatically sending the response, click 'end now' within the banner. Or, if you'd like to edit the response, click 'vacation settings.'
- Log in to your Gmail account.
- Click 'Settings' along the top of any Gmail page.
- From the 'General' tab, select 'Vacation responder on' in the 'Vacation responder:' section.
- Enter the subject and body of your message in the 'Subject:' and 'Message:' fields (you can also use the default message).
- If you've enabled a personalized signature in your account, Gmail will automatically append it to the bottom of your vacation response.
- Check the box next to 'Only send a response to people in my Contacts' if you don't want everyone who emails you to know that you're away from your mail.
- Click 'Save Changes.'
* Messages classified as spam and messages addressed to a mailing list you subscribe to will not receive a vacation response.
- Keep in mind that your vacation response will start over each time you edit it -- if someone receives your initial vacation response, and then emails you again after you'ved edited the subject or body of the message, he or she will receive the edited response, too.