Virtualmin Email Forwarding Issue

I am facing issue while configuring email forwarding. Kindly check below bounce back email

This is the mail system at host

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 (expanded from unknown user: “ztest”

Forwarded Email Address:

Operating System:
Ubuntu 16.04