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

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

Since May 25th, I'm getting bounced e-mails from one of my clients mails.
I get their mails, but 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. 

xxxx@xxx.com, ERROR_CODE :421, ERROR_CODE :, Host not reachable.

Received:from [192.168.15.5] (189.208.55.119 [189.208.55.119]) by mx.zohomail.com

with SMTPS id 1465859708316222.03353870509443; Mon, 13 Jun 2016 16:15:08 -0700 (PDT)

Message-ID:<2993EA52-6484-4083-9698-A9C46765441C@katerinateas.com>

Date:Mon, 13 Jun 2016 18:15:05 -0500

From:Mariana Castro Tamayo 

To:Almacen La Docena Roma 

Subject:factura

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