If you have read receipts that won't send, try changing the SMTP server for the account to a server you can access and change it back to the one you normally use for the account. If they still won't send, you can delete them from the data store by following these steps.
Close
Outlook, download and install
OutlookSpy.
Select the folder you need to delete a hidden item from. For Read Receipts, this is the top level of your message store (Outlook Today folder).
After
selecting the folder, click the IMsgStore button on the OutlookSpy toolbar.
In
the new window, click the Open Root Container button at the top left
of the window to open a new window.
Click the GetContents Table tab. The receipts are listed on the left. Select a receipt, choose Open entry button and a new window opens.
Click
on IMsgStore::Abort Submit. Click OK, close the window. Now choose Delete.
Repeat for each receipt. Close all OutlookSpy windows when you are finished.
Close Outlook and uninstall OutlookSpy.