Split from this thread.
The message could not be sent. The authentication setting might not be correct for your outgoing e-mail [SMTP] server. For help solving this problem, go to Help, search for "Troubleshoot Windows Mail", and read the "I'm having problems sending e-mail" section. If you need help determining the proper server settings, please contact your e-mail service provider.
The rejected e-mail address was '*** Email address is removed for privacy ***'. Subject 'Fw: Welcome to Windows Mail', Account: 'pop.gmail.com', Server: 'smtp.gmail.com', Protocol: SMTP, Server Response: '530 5.7.0 Must issue a STARTTLS command first. x9sm851909igl.10 - gsmtp', Port: 25, Secure(SSL): No, Server Error: 530, Error Number: 0x800CCC78