Google Apps - Requesting Email Read Receipts
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When you are in compose mode, there is a More Options arrow in the lower right corner. Clicking on this will display some additional options for you - one of is "Request read receipt." Clicking on it enables it (places a check next to it). Clicking on it a second time will disable it.
Here's a quick video showing how to turn it on and off: http://www.screencast.com/t/tbP7q99veMV