We have a strange issue with the mail server.

The below error occurs sometimes,
Subject: Undelivered Mail Returned to Sender

This is the mail system at host zimbra.xyz.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<abc@zimbra.xyz.com>: zimbra.xyz.com

DNS records seems to be fine.

Please Advice