Originally Posted by micko_escalade
The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'firstname.lastname@example.org'. Subject 'test66', Account: 'mydomain.com', Server: 'mydomain.com', Protocol: SMTP, Server Response: '554 <email@example.com>: Relay access denied', Port: 25, Secure(SSL): No, Server Error: 554, Error Number: 0x800CCC79
You must authenticate yourself with smtp-auth when you send emails to domains that are not on your server (yahoo.com).
Please check "Server needs authentication" in your outlook express smtp settings.