Mail Delivery Status Notification (Delay) - "The following addresses had fatal errors"

Mail Delivery Status Notification (Delay) - "The following addresses had fatal errors"

Hello,


Since yesterday, I keep receiving this message when sending emails to multiple addresses, including recipients with which I had no problem communicating in the past:

"This message was created automatically by mail delivery system.

THIS IS A WARNING MESSAGE ONLY.

YOU DO NOT NEED TO RESEND YOUR MESSAGE.

The original message was received at Tue, 28 Jul 2015 20:44:08 -0700
from xxxxxx@wwrbt.com [ xxxxxxx@wwrbt.com ]

----- The following addresses had fatal errors -----
[Status: Error, Address: < xxxxxx@ca-cib.com >, ResponseCode 451, #4.1.3 Domain of sender address < xxxxxxx@wwrbt.com > does not resolve]

Message will be retried for 4 more day(s)

----- Transcript of session follows -----

Received:from mail.zoho.com by mx.zohomail.com
    with SMTP id 1438138724620823.5524747666104; Tue, 28 Jul 2015 19:58:44 -0700 (PDT)
Message-ID:<14ed7bf6f98.c354 f529327450.5845794823680481368@wwrbt.com >
Date:Tue, 28 Jul 2015 22:58:44 -0400
From:xxxxxxxxxxx < xxxxxxx@wwrbt.com >
User-Agent:Zoho Mail
To:< xxxxxxx@ca-cib.com >
Subject: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Content-Type:multipart/alternative;
    boundary="----=_Part_311030_1695037345.1438138724246"


Let me know what could be the solution to this problem. Thank you.