'm having a problem sending e-mails with Outllok express. This is what it keep saying.

The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was 'lgdance**@aim.com'. Subject 'test', Account: 'imap.aim.com', Server: 'smtp.aim.com', Protocol: SMTP, Server Response: '556 CLIENT AUTHENTICATION REQUIRED. USE ESMTP EHLO AND AUTH.', Port: 25, Secure(SSL): No, Server Error: 556, Error Number: 0x800CCC78

What can i do to fix this??
Thanks