Subject: Returned mail: see transcript for details

From: Mail Delivery Subsystem <[email-masked]>
Date: Sun, 7 Dec 2025 09:11:10 +0200

The original message was received at Sun, 7 Dec 2025 09:10:57 +0200
from sink.edelkey.net [213.138.147.137]

   ----- The following addresses had permanent fatal errors -----
<[email-masked]>
    (reason: 550 5.4.1 Recipient address rejected: Access denied. For more information see https://aka.ms/EXOSmtpErrors [DB5PEPF00014B92.eurprd02.prod.outlook.com 2025-12-07T07:11:10.575Z 08DE2E754B7C953B])

   ----- Transcript of session follows -----
... while talking to halton-com.mail.protection.outlook.com.:
>>> DATA
<<< 550 5.4.1 Recipient address rejected: Access denied. For more information see https://aka.ms/EXOSmtpErrors [DB5PEPF00014B92.eurprd02.prod.outlook.com 2025-12-07T07:11:10.575Z 08DE2E754B7C953B]
550 5.1.1 <[email-masked]>... User unknown
<<< 503 5.5.2 Need rcpt command [DB5PEPF00014B92.eurprd02.prod.outlook.com 2025-12-07T07:11:10.576Z 08DE2E754B7C953B]