BOUNCED EMAIL : ERROR_CODE :421, ERROR_CODE :, Host not reachable.

BOUNCED EMAIL : ERROR_CODE :421, ERROR_CODE :, Host not reachable.

Hello Team Zoho,

From last week, I'm getting bounced e-mails from one of my clients mails.

I am not getting their mails,also they are not getting mine. I didn't had this problem in the past. 
It's my work e-mail and its crucial that they get this emails. We need constant communication via e-mail. 

This is the bounced email I get in response: 

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. 

xxxxx @xxx.in , ERROR_CODE :421, ERROR_CODE :Host not reachable. 



Received:from mail.zoho.com by mx.zohomail.com 
    with SMTP id 1542863615130188.565742487757; Wed, 21 Nov 2018 21:13:35 -0800 (PST) 
Message-ID:<16739d67499.b1125c20129672 .5036269475682134043@xxx.in > 
Date:Thu, 22 Nov 2018 10:43:35 +0530 
From:LeadBucket <xxxxx @xxx.in > 
User-Agent:Zoho Mail 
To:"Maya Meher" <xxxxx @xxx.in > 
Subject:Fwd: test 
Content-Type:multipart/alternative; 
    boundary="----=_Part_419630_1356079059.1542863615129"


I am requesting you to fix this issue.

Thanks,