“550 5.7. 1 authentication required” indicates reply code, status code and reply text respectively. #Error 550 is an email error code that indicates the user's improper authentication to the server.
#Email errors that start with '5' require an action instead of a retry.
A "550 Authentication is required for relay" error indicates that your email server requires SMTP authentication in order to send outgoing mail, but the email client used to send email has not been authenticated with your username and password.