Would you like a visual indicator that someone is requesting a read receipt?
Download and extract this pst: http://www.poremsky.com/p/files/RRR.zip, then copy the RRR view so that its for "All folders".
To see which messages requested a read receipt, apply the new view you created using the View menu. You'll see a new field in the view - messages with a checked box are requesting receipts. Note that not all messages will display an unchecked (no receipt) box in the field but if they are requesting a receipt, there will be a checked box present.
This will work with all versions of Outlook, regardless of what the very old readme says. Only the steps required to copy the view changed between versions.
For more information and the original information used to create this view, see http://www.slipstick.com/addins/utilities/readrcpt.htm