Thanks for your reply.
The interesting information is near the bottom of here. I have been able to fix it and I think I know where the fault was.
But First
Let’s define a few terms to make sure we know we are talking about the same items:
1) Admin Users, Backend Admin and (site) Administrators: the people in the website’s management team who are meant to receive notice of messages sent through the website.
2) Submitter: The site visitor who populates the form on the contact page in hope of being able to send a message to the Admin Users.
When a contact form is populated and the Submit button is clicked:
1) Admin Users receive a copy of the message
2) The message is recorded in the WordPress database, and
3) The web server logs the outbound email’s path with successful delivery notice received for each recipient.
But,
4) The Submitter does not receive the confirmation email, and
5) The web server does not record the outbound message.
As we can see from 1,2 and 3 the website does successfully send messages by email.
As we can see from 4 and 5, the only addressee to whom the message never arrives is the Submitter. It looks as though the form ignores the confirmation message.
I do receive other automatically triggered notices sent by the website such as security notifications and monthly reports.
The Interesting Stuff – The Fix, Possibly
I’ve switched SMTP plugin. The new one is Post SMTP (www.ads-software.com/plugins/post-smtp/). The test email sent via Post SMTP was successful. Still configured to use the default WordPress email system.
The test email from Post SMTP said it came from wordpress@[THE DOMAIN]. This address is not registered on the server. This made me wonder whether wordpress@ is in use elsewhere within the site’s database and it was.
I did a search and replace in the database using the Better Search and Replace plugin allowing GUIDs to be updated too. I replaced wordpress@ with another address.
Either the fault was fixed by,
A) Switching to the Post SMTP plugin, or
B) The search and replace to change wordpress@ with another contact address
Whichever it was, it worked.
The contact form now sends messages to both admin recipients configured in the form and the Submitter’s email address.
If you want to see the email logs captured by Post SMTP, let me know in a reply to this message and I will send to you through your website’s contact form.
Might I recommend you change the suggested SMTP plugin to Post SMTP.