Error : 550 5.7.51 Service unavailable, Client host(IP address) blocked using Customer Block list AS(1420) in Email Security Cloud

Products

• Email Security.cloud

Issue/Introduction

Track and Trace displays an error message "550 5.7.1 Service unavailable, Client host [IP address] blocked using Customer Block list AS(1420)

Environment

Email Security.cloud

Cause

The Track and Trace logs shows an error stating "550 5.7.1 Service unavailable, Client host [IP address] blocked using Customer Block list AS(1420).

This is a Microsoft rejection error code.

Resolution

• This error indicates that the sender's IP address has been blocked by the recipient's email service provider, likely Microsoft 365.
• Please contact the recipient's admin to delist the sender's IP address from their block list to allow email messages to be delivered from the sending domain.