That error message usually means that you may have the wrong mailserver typed in
your account (SMTP, POP3 addresses) or your mail server may be down. Call up your
ISP and confirm your account settings
You are trying to send emails via an SMTP server that you don't have access to.
Are the email addresses all with the company who who provide your DSL connection. If not thats why the problem has happened.
It may have been fine before, but they may have changed their settings.
Your SMTP server should ALWAYS* be the one of the ISP you are connected to the Internet via and NOT the one that your email ISP provides (unless they are the same)
Can you post the URL for the homepage of the DSL ISP you are using please?
Posting that error first time round would have been far more helpful...learn from your mistakes.
*NB Always always has exceptions.