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 (ntlworld.com), but not to other addresses e.g
aol.com. I get the message.
An unknown error has occurred. Subject 'Test', Account: 'pop.ntlworld.com',
Server: 'smtp.ntlworld.com', Protocol: SMTP, Server Response: '550 you are
not allowed to send mail to <*****@aol.com>', Port: 465, Secure(SSL): Yes,
Server Error: 550, Error Number: 0x800CCC69
.