You received a Non-Delivery Report (NDR) for email sent through the Symantec Email Security.cloud infrastructure, with the message "You are trying to use me [server-X.tower-XXX.messagelabs.com] as a relay, but I have not been configured to let you [IP, address] do this."

453-you are trying to use me [server-X.tower-XXX.messagelabs
453-.com] as a relay, but I have not been configured to let
453-you [IP, server.address] do this. Please
453-visit https://knowledge.broadcom.com/external/article?articleId=169652
453-for more details about this error message and instructions to
453-resolve this issue

When the email is sent from a customer domain through the Symantec.cloud outbound to a 3rd party domain, check the following:

• If your organization uses Office 365 or Google G Suite to host email. This will authorize the email security service to accept and scan outbound email from these IP addresses.
  
  See Configure Google G Suite for outbound email
  See Configure Microsoft Office 365 and Symantec.cloud for outbound mail
   
• If your organization sends email from on-premise mail servers, please register the on-premise gateway IP addresses. This will authorize the email security service to accept and scan outbound email from these IP addresses.
  
  See Configuring Microsoft® Exchange 2007 and 2010 for outbound mail

Additional Information

1. If you recently made changes to the Outbound Routes configuration, allow approximately 1 - 2 hours for this to take effect and for the Symantec.cloud infrastructure to start using the new settings which allow your traffic through.
    
2. Verify that the Smart Host (relay) address you have configured on your email service (or platform) is correct, according to the one assigned to you during the account provisioning (e.g.: cluster1out.eu.messagelabs.com). The outbound address generally matches the inbound MX record address of the same domain.
    
   • The IPs associated with the cluster addresses can change without prior notice, we strongly recommend against using specific IPs or other forms of addressing for relaying outbound emails through the service.
      
3. When the email is sent inbound to a domain belonging to a customer of Symantec.cloud:

• Verify that the IP address you are trying to deliver the email to corresponds to the MX record cluster address for that domain.
• Verify that the MX record address registered for the given domain is the correct one, which was provided when the domain was first provisioned.
• Mail eXchanger (MX) record setup with Email Security.cloud
• Troubleshooting MX records during inbound delivery failure

4. Ensure that the Symantec.cloud service is active. This can be checked by attempting to log into the service portal. If you cannot, it is possible that the account was suspended. To confirm regarding this possibility, contact our technical support team.