I don't use Outlook for e-mail, I use OE.
I assume that 'Read Receipts' are the same in both programs.
If you request a Read Receipt in OE, it is up to the recipient of your e-mails to click on 'Yes' when he gets your e-mail.
Only then will you actually receive anything back to verify receipt.
If the recipient clicks 'No' you won't receive a Read Receipt.