Diagnostic-Code: 5.4.4 DNS error:NXDOMAIN. Domain not found

Diagnostic-Code: 5.4.4 DNS error:NXDOMAIN. Domain not found

Hi, I am getting this error . 
Both Sender and Recipients  email servers are hosted at Zoho. 
All MX record as set well.
we can send from other hosting server.

-----Original Message----- 
From: mailer-daemon@mail.zoho.com <mailer-daemon@mail.zoho.com> 
Sent: Monday, October 19, 2020 3:40 PM 
To: XXXXX@mm-ix.net 
Subject: Undelivered Mail Returned to Sender 

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. 

'XXXXXXX@optimitycomm.com', ERROR CODE :512 - 5.4.4 DNS error:NXDOMAIN. Domain 
not found :optimitycomm.com' 



Reporting-MTA: dns; mx.zohomail.com 
Arrival-Date: Mon, 19 Oct 2020 15:40:13 +0630 

Original-Recipient: rfc822; 'XXXX@optimitycomm.com' 
Final-Recipient: rfc822; 'XXXXX@optimitycomm.com' 
Status: 512 
Action: failed 
Last-Attempt-Date: 19 Oct 2020 09:10:20 GMT 
Diagnostic-Code: 5.4.4 DNS error:NXDOMAIN. Domain not found :optimitycomm.com'