Can't send e-mails

Can someone help? I can receive e-mails and send e-mails to people with the
e-mail address like mine (, but not to other addresses e.g I get the message.
An unknown error has occurred. Subject 'Test', Account: '',
Server: '', Protocol: SMTP, Server Response: '550 you are
not allowed to send mail to <*****>', Port: 465, Secure(SSL): Yes,
Server Error: 550, Error Number: 0x800CCC69