PROBLEM: Getting 'Unknown Error 0x80040201' when sending email in Outlook? 

SITUATION: An Outlook sending error pops up with the above number in it when the email application tries to send a message that you can't see anywhere.
           All messages are sent and received properly; but the error message pops up on its on or when sending regardless.

CAUSE: When Outlook tries to send a read receipt to the sender and the address doesn't exist, this error is generated. The only way to fix this is to 
       use Outlook Spy, as described in http://www.outlook-tips.net/howto/delete_rr.htm

SOLUTION: Go to the above web page (in CAUSE:) and follow the directions. It tells you to download and install Outlook Spy from 
          http://www.dimastr.com/outspy/. Do that, and follow the rest of the instructions in http://www.outlook-tips.net/howto/delete_rr.htm to delete
          the read receipt that is giving you this error. At the end of the process you will uninstall Outlook Spy. It worked for me!!!!