Here, found this:
0x800ccc0f : (often misread as 0x800cccOf) The SMTP server didnt open up for sending. Make a read mail call right before trying to send. See We do not relay
Error 571: Name@Mail.Net
prohibited. We do not relay is an error message by the SMTP server. If you dial into the Internet with service B and try to send with service A the SMTP server of A may not have you identified and refuses to send mail from an unknown. Remedy: Try the process immediately again, as the pop server may have been successfully opened (with password) by Outlook and informed its SMTP collegue that you are OK. SMTP then opens up for you for a couple of minutes. Or: use the trick with the two identical accounts.
451Need to authenticate via SMTP-AUTH-Login tells you that the SMTP server didnt open up for sending, see we dont relay above.
If Outlook ever asks you to repair the mail inbox, just look for and use scanpst.exe on your PC.