INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :5.7.1

guys, what can i do? this is keep happening randomly with one user of mine,

pd. im alredy add the spf record

pd2. im using aws to host the domain

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.

mmendez@kosiuko.com INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :5.7.1 Recipient not authorized, your IP has been found on a block list

Received:from mail-lf0-f43.google.com ( mail-lf0-f43.google.com [209.85.215.43]) by mx.zohomail.com
with SMTPS id 1464804265963985.263855650492; Wed, 1 Jun 2016 11:04:25 -0700 (PDT)
Message-ID:<CAG6P= mSJi_nPNkju9DR5eKy8ZapouTfYyogCedgegm58L+0mQg@mail.gmail.com >
Date:Wed, 1 Jun 2016 15:04:22 -0300
From:Gabriel Garcia < Gabriel@laskaprinas.com.ar >
To:Manuela < mmendez@kosiuko.com >
Subject:Cheques Verano
Content-Type:multipart/ alternative; boundary= 001a1141eeb8bff90605343b513d