Subject: Undelivered Mail Returned to Sender
From: [email-masked] (Mail Delivery System)
To: [email-masked]
Date: Sun, 21 May 2023 19:52:33 +0200 (CEST)

This is the mail system at host mx4.cloud4u.solutions.

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 postmaster.

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 gmail-smtp-in.l.google.com[74.125.71.27] said:
    550-5.7.26 This mail is unauthenticated, which poses a security risk to the
    550-5.7.26 sender and Gmail users, and has been blocked. The sender must
    550-5.7.26 authenticate with at least one of SPF or DKIM. For this message,
    550-5.7.26 DKIM checks did not pass and SPF check for [duerring.org] did
    not 550-5.7.26 pass with ip: [217.7.157.11]. The sender should visit
    550-5.7.26  https://support.google.com/mail/answer/81126#authentication for
    550 5.7.26 instructions on setting up authentication.
    l14-20020adfe58e000000b00307ae5e85c8si2057248wrm.728 - gsmtp (in reply to
    end of DATA command)