Mail Delivery Issue Due to Loopback Address Delivery

Products

• Email Security.cloud

Issue/Introduction

Email Security.Cloud was unable to deliver the email to the recipient due to the following error:

551 5.4.6 [internal] Private/Loopback Address

Environment

Email Security.Cloud

Cause

This issue occurs when the recipient domain MX record is pointed to a private IP address, such as 10.x.x.x, or a loopback address, like 127.0.0.1.

 

Resolution

Check the MX records of the recipient domain to ensure that it is configured correctly and not pointed to a loopback address. This can be verified using an external tool such as MX Toolbox.