I am a Zoho mail user with an email address *@kukuruyuk.com

Recently I keep getting a lot of undelivered mail with ResponseCode 421 Host Not Reachable

The domain which we failed to send the emails are:

 *@jne.co.id

 *@transretail.co.id

 *@mitra10.com (undelivered email response is attached)

In the past, we've had numerous successful email sent to those domain. But suddenly in the past week, the emails weren't delivered. Is there anything we can do to fix this issue?