When I try and send an email with Outlook, I get the following error - “Unknown Error 0x80040201”. Not very helpful. We used to use Exchange, now we're using a POP3 server (don't laugh).
Luckily this post pointed out the solution...
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.
Thanks, Ben!