From: Mail Delivery Subsystem <mailer-daemon@example.net>
To: sender@example.com
Subject: Delivery Status Notification (Failure)
Date: Tue, 02 Jun 2026 10:00:00 +0000
Message-ID: <hard-bounce@example.net>
Content-Type: text/plain; charset=utf-8

Delivery failure.
Final-Recipient: rfc822; missing@example.com
Action: failed
Status: 5.1.1
Diagnostic-Code: smtp; 550 5.1.1 User unknown