Your message was refused for sending.
WHAT IT MEANS
The server refused to pass your message to an external recipient because it does not recognise you as authorised.
THE CAUSE
Outgoing authentication is not enabled in your mail client.
Without it, the server sees an anonymous connection attempting to send to the outside world, which is exactly what a spammer does.
THE FIX
In your mail client's outgoing server settings, enable authentication and supply the same username and password as incoming.
Where the client offers "Use same settings as incoming server", 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 is the prefix rather than the full email address.
WHY THE RESTRICTION EXISTS
An open relay, which sends for anyone, is found and abused within hours.
Every properly configured mail server requires authentication.
THE MOST COMMON CLIENT ERROR
iPhone and iPad leave the outgoing username and password blank because iOS labels them optional. They are not.
