If you're seeing 550 Authentication Required when sending emails, it means the server has not been correctly configured to receive emails from that domain.

The two top causes for this error are:

• Your MX records are not up to date or fully propagated. Please follow our tutorial to find your DNS records and copy them over to your DNS manager.
• Email sending is disabled for that domain. Ensure you have selected “Use this server to handle my e-mails” inside the “Email Routing” section.