I've solved the problem. I mean to use this post to share
the solution to others who have had this problem. It
happens when you import email into outlook and reply to
one such email. If you do not have the person that you are
replying to listed in your contacts, this error can occur.

The solution is simple:

1. Open your outbox
2. Double click the first email that is unsent
3. Change the email address from the alias such as 'Joe
Error' to the actual address that looks something like
joe_error@here.com.
4. Repeat for all the emails in this folder.
5. Relax! It is fixed.


>-----Original Message-----
>When using microsoft outlook I came across a strange
error
>message. All of the sudden, I was unable to send emails
>through my pop3 account. Outlook reported "Unknown Error
>0x80040201". Of course, I thought it must be the email
>servers at fault. That is not the case, I've tried
several
>other pop3 accounts and my own hotmail account. I've also
>reinstalled outlook on my computer. Nothing seems to
work.
>I searched on the web to see if anyone else had the
>problem, and a few did. However, only one source had a
>solution. 'Purchase a subscription to X tech support web
>site and we will tell you how to fix it'. I'd sooner
>format my hard disk and reinstall everything from scratch
>than pay $20 dollars for someone to profit from my
>misfortune! Has anyone else had this problem?
>.
>