Help with: Unable to send message;Reason:550 5.1.0 Authentication Required

Help with: Unable to send message;Reason:550 5.1.0 Authentication Required

 
Hi
Have just set up Zoho mail - and all tested ok - emails receive
When I try to send from ZohoMail it always pops up with "Unable to send message;Reason:550 5.1.0 Authentication Required"
Can you please assist?