Working for months, but cannot send to one specific address.  Other addresses work.  I CAN send to the problem address from a private aol.com email account, just not from mail.zoho.com.  Error message is (I removed part of my customers email address for their privacy)

...associates@comcast.net INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :5.1.1 Not our Customer



Received:from mail.zoho.com by mx.zohomail.com
    with SMTP id 1473775704798567.2116140447395; Tue, 13 Sep 2016 07:08:24 -0700 (PDT)
Message-ID:<15723e09ad7.baa8cb1a150569.9074646621277495473@hipaasimple.com>
Date:Tue, 13 Sep 2016 09:08:24 -0500
From:Marion Neal <marion@hipaasimple.com>
User-Agent:Zoho Mail
To:"...associates" <...associates@comcast.net>
Subject:To Sally - Here is the info, and thank you
Content-Type:multipart/alternative;
    boundary="----=_Part_461403_153166292.1473775704796"