You may notice that your Outlook MAPI clients don't receive read receipts when transmitting their messages over the Exchange 5.5 Internet Mail Service (IMS), even when sending the messages to another MAPI client. IMS is the reason for this: It converts the read receipt property into the wrong MAPI property at the receiving end.
For some users, this behavior is annoying. However, you can force IMS to preserve all of the Internet and MAPI properties when transmitting messages.
Follow these steps:
The Outlook client at the receiving end should now receive the correct MAPI property, and it should properly generate the read receipt.