Mail delivery failed: returning message to sender

Hi Ray,
I wondered if you could make any sense of this message I received
I cannot understand where the address john@net100.xo.uk has come from.

The following address failed:
 
john@net100.xo.uk
 
For the following reason:
 
multiple delivery attempts failed
 
--- The header of the original message is following. ---
 
Received: from localhost ([82.165.80.141]) by mrelayeu.kundenserver.de
(mreue010 [213.165.67.97]) with ESMTPSA (Nemesis) id
1MaIrN-1ldpZT1uq5-00WEQy; Wed, 07 Jul 2021 13:18:38 +0200
To: john@net100.xo.uk
Subject: Thank you for sending your boat advertisement
Date: Wed, 7 Jul 2021 13:18:37 +0200
From: info@boatsforsalenow.com
Cc: john@net100.co.uk
Message-ID: <u6KzRINPBRSLqav4ZIW5lglr77QLCQQpoFsVbQmSeA@boatsforsalenow.com>
X-Mailer: PHPMailer 6.0.3 (https://github.com/PHPMailer/PHPMailer)
MIME-Version: 1.0
Content-Type: multipart/alternative;

Somebody probably entered a bad email address when filling out your online form. Find the email message you send with the subject: "Thank you for sending your boat advertisement"

That is the page that the user entered a bad email address on.

Hello Ray
Please see pm

Please attach a copy of the .php page that has the email server behavior applied and I'll take a look to see if I can spot anything.

Hello Ray
I am attaching the two pages which contain email entry boxes

Neither of those two pages send an email that I can see. Maybe it is sent later in the process? On the next page where the images are uploaded or on a confirm page?

I am sending two more attachments here with more as the file names are the same in the next folder

More attachments here

None of these pages send an email. The issue has to be on a page that sends an email... The message does imply that the email is sent from your site, but it isn't from one of these pages.

Search the pages for email subject "Thank you for sending your boat advertisement" and you will see that it isn't referenced on any of these pages at all. Maybe do a site wide search for that subject to find the page with the email code.

Hello Ray,
I am attaching an image which I think shows that some files have been lost. The strange thing is that the site was working OK for a short time.
Regards
John