Question about Outlook Express

I am unable to send emails when using outlook for certain email addresses. (cablevision) I can recieve email. When I send an email I recieved the following error message. "Server error: '550 5.7.1  Your privileges to use the outbound Mail service have been suspended, error code 81". I have been in contact with the email provider and they stated that it is not their issue as I can send and recieve email via the web email option. Thank you for any advice.