Subject: Returned mail: see transcript for details

From: Mail Delivery Subsystem <[email-masked]>
Date: Tue, 14 Oct 2025 11:30:48 +0300

The original message was received at Tue, 14 Oct 2025 11:30:35 +0300
from gate.edelkey.net [213.138.147.140]

   ----- 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 [AMS0EPF000001B1.eurprd05.prod.outlook.com 2025-10-14T08:30:48.372Z 08DE08442E09E0B8])

   ----- 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 [AMS0EPF000001B1.eurprd05.prod.outlook.com 2025-10-14T08:30:48.372Z 08DE08442E09E0B8]
550 5.1.1 <[email-masked]>... User unknown
<<< 503 5.5.2 Need rcpt command [AMS0EPF000001B1.eurprd05.prod.outlook.com 2025-10-14T08:30:48.372Z 08DE08442E09E0B8]