A rejection when sending, and almost always a configuration fault rather than a server problem.
WHAT IT MEANS
The server refused to pass your message on to an external recipient, because it does not recognise you as an authorised sender.
THE CAUSE
Outgoing authentication is not enabled in your mail client. Without it the server sees an anonymous connection trying to send to the outside world, which is exactly what a spammer does, and refuses.
THE FIX
In your mail client's outgoing server settings, enable authentication and supply the same username and password as the incoming server.
Where the client offers "Use same settings as incoming", tick it.
OTHER CAUSES
The From address does not match the mailbox you authenticated as. Sending as a different address is treated as relaying. The username entered is the prefix rather than the full email address.
WHY THIS RESTRICTION EXISTS
An open relay is a server that will send mail for anyone. They are found and abused within hours. Every properly run mail server requires authentication.
