Mail Not Delivered

Hi Team,

When I am trying to send email to recipients through Zoho help desk, But it doesn't deliver and it returns the below error message.

mailer-daemon@mail.zoho.com

<support@optisolbusiness.zohosupport.com>

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 Mon, 18 Sep 2017 06:31:21 -0700
from support@optisolbusiness.zohosupport.com [ support@optisolbusiness.zohosupport.com]

----- The following addresses had fatal errors -----
[Status: Error, Address: < anitha.b@optisolbusiness.com>, ResponseCode 421, Host not reachable., Status: Error, Address: < jai@optisolbusiness.com>, ResponseCode 421, Host not reachable., Status: Error, Address: < balachandran.s@optisolbusiness.com>, ResponseCode 421, Host not reachable.]

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 150573872445749.35215214047628; Mon, 18 Sep 2017 05:45:24 -0700 (PDT)
Message-ID:<f14564b31679a112aab53dd1094e5dba91e21925ef6ee0bf6f31 42116ba97f52_2ma8u_15e95058fe8@zohodesk.com>
Date:Mon, 18 Sep 2017 05:45:24 -0700
From:Zoho Support < support@optisolbusiness.zohosupport.com>
User-Agent:Zoho Mail
To:< jai@optisolbusiness.com>
Cc:" balachandran.s@optisolbusiness.com" < balachandran.s@optisolbusiness.com>,
    "anithaletchumi b" < anitha.b@optisolbusiness.com>,
     < support@optisolbusiness.zohosupport.com>
Subject:Re:[## 355 ##] Install MS-Excel-High
Content-Type:multipart/alternative;
    boundary="----=_Part_74131_349788973.1505738724455"

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