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 user@domain.tld . Subject 'Subject', Account: 'mail.domain.tld', Server: 'mail.domain.tld',
 Protocol: SMTP, Server Response: 451 user@domain.tld 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.

Solution:

• Download the email first to get authenticated, don't just click on the “Send/Recv” button on Outlook Express. Instead, click and hold on the “Send/Recv” button, select “Receive All” first.
• Or, you may turn the SMTP authentication ON in your profile and use port 587.