deleting an item which can't be sent

I accidentally pressed OK to send a receipt confirming
message had been read to someone who had sent me an E mail

Unfortunatly the ISP I am registered on is not the same
as the one that used in the E mail address .Outlook keeps
trying to send a message using the non valid isp
resulting in an error message. I can change accounts OK
if I reply to a non valid ISP by using the drop down menu
but I can't seam to get rid of the attempt to send the
receipt using a non valid isp.

Any ideas how to delete the confirmation of the message
having been read??