Subject: Undelivered Mail Returned to Sender
From: [email-masked]
To: [email-masked]
Date: Sun, 31 Jan 2021 07:31:11 +0100 (CET)

This is the mail system at host cernah01.cern.ch.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to [email-masked].

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<[email-masked]>: host cernmxgwlb4.cern.ch[188.184.36.53] said: 550 5.1.1
    User unknown (in reply to RCPT TO command)