The error message "550 Authentication is required for relay" means that the email server requires authentication before allowing email to be relayed. When someone tries to send an email via the server, they must provide valid credentials to verify their identity before the server will allow the message to be sent.
If you're seeing this message when trying to send email via our mail server, check that your settings are correct and that you're authenticating with your username and password.
This is a security measure to prevent unauthorised use of our email server for sending spam or other harmful messages. Authentication ensures that only authorised users are permitted to send email through the server. If you receive this message, it means you need to enter the correct username and password to send email via the server in question.