Subject: Mail delivery failed: returning message to sender
From: "Mail Delivery System" <[email-masked]>
To: <[email-masked]>
Date: Sat, 16 Mar 2024 17:46:54 +0330

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. The following address(es) failed:

  [email-masked]
    host extmail.optusnet.com.au [211.29.133.14]
    SMTP error from remote mail server after RCPT
TO:<[email-masked]>:
    550 5.1.1 <[email-masked]>: Recipient address rejected:
    User unknown in virtual mailbox table