To reply to all email with a fixed response as soon as it arrives, click Options, and then Define Rules. Scroll down to the section 'Automatic notification/forwarding' and check the 'Enable vacation message' box. Type your auto-reply message in the 'Vacation message text' box.
Please ensure that you check in 'Personalities' that you have included all the email addresses you use in the 'Send From' section, since only messages to these addresses result in vacation replies being generated. Also, please note that only one reply will be sent to each sender address, for 3 days following the first vacation message. This is to ensure that people sending messages to you do not get multiple identical auto-replies from you every day (which would get very annoying). This does however make it a little hard to test your own auto-reply, since once you have tested it successfully once, you will get no further replies for 3 days.