For 5 days I have tried to send an update email to about 40 people (for a golf tournament). I have sent 2 emails to this list of people within the past month. Outlook would at first fail to send and then also not allow me to save my email (very frustrating). I eventually rebuilt the email and addressee list and sent again, receiving immediately an unknown send error 0x80040201 except I did not notice there was an error until the next day when I saw the email in my outbox. I tried many things suggested on various forums and nothing worked. Finally, I sent the email out to subset of the to list until it worked and discovered there was a single email address that caused a problem. I have received email from this exact email address, so what caused the problem? The person recently remarried, so I edited her contact info to reflect her new last name, even though the actual email address did not change. Somehow this caused Outlook to use some funky string of text for the email address and it immediately fails without sending the email to anyone else on the To list. I solved the problem by creating a new contact. Very frustrating. I had no way to know the issue was with one of the email addresses because when I would list the contact info for anyone in the To list, it looked fine. It was only after hitting "send" that it translated to some nonsense for that one person. I post this in case this may help someone else who runs into this problem or in case MS wants to try to fix the bug.