Email error code : 551 5.4.6 [internal] Private/Loopback Address

Products

• Email Security.cloud

Issue/Introduction

You are getting an error code when sending to end recipient, generated error : 551 5.4.6 [internal] Private/Loopback Address

Environment

Email Security.cloud

Cause

The issue occurs when the recipient domain is pointing to an MX binding it to a private IP address 10.x.x.x or  a Loopback address 127.0.0.1

Resolution

The recipient's Admin must ensure that their MX are resolving correctly to a valid Public Address.