This is a fairly common error message. While normally associated with Outlook Express, you may also see this error in Outlook.
|An unknown error has occurred. Subject 'example', Account: 'firstname.lastname@example.org', Server:
'mail.example.com', Protocol: SMTP, Port: 587, Secure(SSL): No, Error Number: 0x800CCC0B
The error occurs when you attempt to send an e-mail, but your outgoing mail settings in Outlook Express do not include your user name and password. To correct this, you will need to enable authentication for outgoing mail. Guides on how to check your outgoing mail settings are available here.