The easiest way to create an autoreply for an individual address is to use the filters in your webmail. Once logged in, click the "filters" link that appears at the right end of the menu. A list of current filters is shown.
Adding an autoreply
Click on the "Add a New Rule" button and follow through the wizard.
First select all messages (if you have a spam filter already this will not autoreply to spam as the spam filter tells the server to stop once it's moved the spam to the Spam folder):
Move to the next step and select the "Vacation" option and fill out the message you wish to have sent back to the sender.
Move to the next step and confirm that the details are correct, then press Finish.
You are then shown the list of filters. This is not yet permanent, you need to click "Save Changes" for them to be made active.
Changing an autoreply
Click on the edit icon on the right side of the filter you wish to change. The details are shown and can be modified. Click "Apply Changes" once you've made your changes and then click "Save Changes" to get them saved to the server.
Deleting an autoreply
Click on the delete icon on the right side of the filter you no longer want. To make the delete permanent you need to click "Save Changes".