Error 550.5.7.1

Error 550.5.7.1

I get an error 550 5.7.1 Access Denied error when I send emails to particular donains namely @ecopetrol.com.co any ideas why?

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 A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.

xxxxx@ecopetrol.com.co INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :5.7.1 <xxxxxxx@ecopetrol.com.co>: Recipient address rejected: ERS-RBL.



 Received:from  mail-pg0-f50.google.com  ( mail-pg0-f50.google.com  [74.125.83.50]) by  mx.zohomail.com
        with SMTPS id 1502827839291451. 9247331905144; Tue, 15 Aug 2017 13:10:39 -0700 (PDT)
 Message-ID:< CALYwuVJGQ7gKmN8f5D1BojrpMrz-M28OPgPzmG-VYoefbMdEzQ@mail.gmail.com >
 Date:Tue, 15 Aug 2017 15:10:38 -0500
 From:Stiven Parrado < xxxxx@esetesas.com >
  To:xxxxxx@ecopetrol.com.co
 Subject:Prueba
 Content-Type:multipart/ alternative; boundary=" 94eb2c14847e7cbf890556d05f8e"