Tienes dudas si una notificación de correos fallidos que te llego por correo es real?
La mayoria de los hackers estan buscando como engañarte para que pongas tus datos de correo y envian cosas como las siguientes

Nombre de tu dominio
Your password for email is about to expire on 28 August, 2024 08:42:31 AM.
You may visit www.domain.com/account/Nora.r/activity/ to see your recent activities.
© 2024 DOMAIN All rights reserved
Este tipo de correos de llama phishing y es una técnica de ingeniería social o engaños que utilizan los ciberdelincuentes para obtener información personal y financiera de los usuarios de forma fraudulenta
Los mensajes de regreso llegan 1 a uno No necesitas actualizar contraseñas de ningun tipo y en el mismo mensaje se detalla cual fue el problema sin tener que visitar sitios o URLs
Ejemplo real de un mensaje de regreso o bounce back
Return-path: <>
Envelope-to: sender@example.com
Delivery-date: Thu, 25 Oct 2012 16:42:54 -0400
Received: from mailnull by servername with local (Exim 4.77)
id 1TRUGT-0005Qd-RT
for sender@example.com; Thu, 25 Oct 2012 16:42:54 -0400
X-Failed-Recipients: recipient@RemoteDomain.com
Auto-Submitted: auto-replied
From: Mail Delivery System <mailer-daemon@ecbiz103.domain.com>
To: sender@example.com
Subject: Mail delivery failed: returning message to sender
Message-Id: <e1trugt-0005qd-rt@ecbiz103.domain.com>
Date: Thu, 25 Oct 2012 16:42:53 -0400</e1trugt-0005qd-rt@ecbiz103.domain.com></mailer-daemon@ecbiz103.domain.com> 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: recipient@RemoteDomain.com
No Such User Here —— This is a copy of the message, including all the headers. —— Return-path: <sender@example.com>
Received: from sender@example.com by servername with local (Exim 4.77)
(envelope-from <sender@example.com>)
id 1TRUGT-0005QT-Q5
for recipient@RemoteDomain.com; Thu, 25 Oct 2012 16:42:53 -0400
To: recipient@RemoteDomain.com
Subject: [example.com] Please moderate: “About us”
X-PHP-Script: example.com/wp-comments-post.php for 123.123.123.123
Date: Thu, 25 Oct 2012 20:42:53 +0000
From: WordPress <wordpress@example.com>
Message-ID: <82f2c6cb759fb7e7c03356a25c799790@example.com>
X-Priority: 3
X-Mailer: PHPMailer 5.1 (phpmailer.sourceforge.net)
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset=”UTF-8″ A new comment on the post “About us” is waiting for your approval
https://example.com/ Author : Payday loans (IP: 123.123.123.123)
E-mail : payday@spam.com
URL : https://www.paydayspam.com
Comment: Payday loans are available for any occasion!