"550 Authentication is required for relay" indicates that the email server requires authentication before allowing the relay of email messages. When someone attempts to send an email through the server, they must provide valid authentication credentials to confirm their identity before the server permits them to send the message.
Make sure to check outgoing settings in your email program that you authenticate with username and password before sending.
This is a security measure to prevent unauthorized use of the email server for sending spam or other malicious messages. Authentication helps ensure that only authorized users have the right to use the server for sending emails. If you encounter this message, it means you need to enter the correct username and password to be able to send emails through the respective server.