unable to send emails from zoho to another domain

unable to send emails from zoho to another domain

Hi, i am using zoho email and i am unable to send a email to a user on another domain. Its only happening for zoho and i am receiving the following bounce back.

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 Wed, 28 Feb 2018 07:12:02 -0800
from shan.ferouze@zoho.com [ shan.ferouze@zoho.com]

----- The following addresses had fatal errors -----
[Status: Error, Address: < ferouze.padrudeen@aqaar.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 1519827998001833.7086717805304; Wed, 28 Feb 2018 06:26:38 -0800 (PST)
Message-ID:< 161dcced52f.c37d49bf14108.3453241559411519835@zoho.com>
Date:Wed, 28 Feb 2018 18:26:37 +0400
From:ferouze deen < shan.ferouze@zoho.com>
User-Agent:Zoho Mail
To:"ferouzepadrudeen" < ferouze.padrudeen@aqaar.com>
Subject:tttt
Content-Type:multipart/alternative;
boundary="----=_Part_36441_1380118372.1519827998000"

Kindly help