Invisible Read Reciept stuck in Outbox

I recently started working from home. As a security
measure, my work email (set up through Outlook) can
receive from but not send to external email addresses.
When I was in the office, I had it set up to send read
receipts without asking me about it. Since I can no
longer send to external email addresses, however, there
are now two read receipts stuck in my Outbox that will
not send. It would be easy enough to delete them except
that they do not show up in the Outbox! I only know they
exist because of the error I receive when trying to send.

This is Outlook 2003. Can anyone help me get rid of these
invisible read receipts??