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Receiving the #550 Unable to relay error when sending an email

Receiving the <server> #550 Unable to relay error message for outbound emails indicates that authentication has not been configured.

Outbound emails are authenticated in one of two ways:

  • Server based (By IP address) - The external IP addresses of your sending servers must be added to the Email Security product.

    To do this, go to Product Configuration > Outbound Mails and make sure that each external IP address that the server will be connecting to EMS with is added to the list. For information on how to add an IP address refer to this kb article.


    When adding IP address to this list, Email Security will only route emails which originate a from address from the Domains section.

    For Office 365, an SPF record is added to the Outbound Mail hosts. Messages will be rejected unless the following requirements are met:

    • The IP address of the sender comes from an Office 365 server listed in the SPF record.

    • The mail from: or from: in the headers comes from a domain that is configured for the customer in the Domains section.


    For more information on configuring Office 365, please refer to Configure Office 365 for GravityZone Security for Email.Configure Office 365 for GravityZone Security for Email

  • User based (SMTP Authentication)

    This method is only used when the user is connecting directly from a dynamic IP address. SMTP authentication is done by providing a username and password.

    You will need to log a case with your Service Provider to have SMTP Authentication enabled for these users.


    By Default, this feature is not enabled. Please contact your service provider for further assistance.