From: Sender Verification <verify@example.net>
To: sender@example.com
Subject: Sender verification required
Date: Tue, 02 Jun 2026 10:09:00 +0000
Message-ID: <challenge-response@example.net>
Content-Type: text/plain; charset=utf-8

This is a challenge-response message. Please verify yourself before your message
can be delivered to challenge@example.com.