The following shows the typical error from the SMTP session using mail client such as Outlook or Outlook Express:
The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was firstname.lastname@example.org . Subject 'Subject', Account: 'mail.domain.tld', Server: 'mail.domain.tld', Protocol: SMTP, Server Response: 451 email@example.com please authenticate before send', Port: 25, Secure(SSL): No, Server Error: 451, Error Number: 0x800CCC78
The error occurs due to user was trying to send the email out without being authenticated first by downloading with POP3 or login with Webmail.