ERROR_CODE :Delivery not authorized, message refused (ERROR 571)

ERROR_CODE :Delivery not authorized, message refused (ERROR 571)

Hello,

I cannot send emails to addresses that worked fine last week.  I get the following message when sending messages to any address @elmundo.com.sv :

This message was created automatically by mail delivery software. 
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. 

raymundo@elmundo.com.sv Error, ERROR_CODE :571, ERROR_CODE :Delivery not authorized, message refused 

Received:from mail.zoho.com by mx.zohomail.com 
    with SMTP id 141503161646865.31731186942329; Mon, 3 Nov 2014 08:20:16 -0800 (PST) 
Message-ID:<1497674f396.fc 3c4a9336135.654856974789265429@elmundo.sv > 
Date:Mon, 03 Nov 2014 10:20:16 -0600 
From:Juancarlos Burmester < jburmester@elmundo.sv > 
User-Agent:Zoho Mail 
To:"Eugenio Raymundo" < raymundo@elmundo.com.sv > 
Subject:Prueba de envío 
Content-Type:multipart/alternative; 
    boundary="----=_Part_47363_1079522870.1415031616398"

Is it possible to white flag the destination domain to stop this from happening?  I'm new to Zoho and I am evaluating their service in order to migrate our mail server, currently managed on premises.

Any comments?