Test works, but mails sent through WordPress fail with Gmail smtp

Here is the error of the bounce.

Reporting-MTA: dns; googlemail.com
Received-From-MTA: dns; [email protected]
Arrival-Date: Mon, 10 Jan 2022 19:35:09 -0800 (PST)
X-Original-Message-ID: <[email protected]>

Final-Recipient: rfc822; [email protected]
Action: failed
Status: 5.0.0
Diagnostic-Code: smtp; Message rejected.  See https://support.google.com/mail/answer/69585 for more information.
Last-Attempt-Date: Mon, 10 Jan 2022 19:35:09 -0800 (PST)

All domains and emails are fake here for posting

Here is the header of the bounced email (domains and email addresses changed…)

X-Google-Smtp-Source: ABdhPJwVviEaRiSzKiYEB5DXmYZOeiKzZlO6qgbbD+Y9ZdpMTW9ZqtORo9l88nA5jfO8RD39g7Tbpg==
X-Received: by 2002:a05:6830:926:: with SMTP id v38mr2081644ott.121.1641872109448;
        Mon, 10 Jan 2022 19:35:09 -0800 (PST)
Return-Path: <[email protected]>
Received: from subdomain.mydomain.com (gatorxxx.hostgator.com. [198.xx.xx.xxx])
        by smtp.gmail.com with ESMTPSA id c29sm1826032otd.14.2022.01.10.19.35.08
        for <[email protected]>
        (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256);
        Mon, 10 Jan 2022 19:35:09 -0800 (PST)
Date: Tue, 11 Jan 2022 03:35:08 +0000
To: [email protected]
From: Tom <[email protected]>
Reply-To: Tom <[email protected]>
Subject: Thank you for the purchase of the product
Message-ID: <[email protected]>
X-Mailer: PHPMailer 6.2.0 (https://github.com/PHPMailer/PHPMailer)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8

Hi, please check the following support thread.

https://www.ads-software.com/support/topic/smtp-error-failed-to-connect-to-server-9/#post-14652405

Let me know if the information above helps you.

Thank you.

It all got resolved today….
It took 4 hours of Google support and then an escalation to a higher level tech…
What finally was the solution was to change the subject of the email…
It was set to “Notificati0on of Product Sale”
That was enough to stop the gmail SMTP server from sending my messages.
Three days of working with gmail on this….

That is good news ??

Does your settings work with 2FA security enabled in your Gmail account?

Thank you.

Yes two factor authentication works.
The setup is to use the “Other smtp”. I did not use the Gmail setup you provide.
During this whole process, I learned a lot.
You can use your gmail address and password as log in parameters or you can use your gmail address and then set an App password. You must allow “Less secure Apps”
What the gmail setup does not tell anyone is that you must also manage a bunch of DNS entries if you use your domain.
So you must set MX records and SPF records. DMARC is recommended as well.
A lot will depend on the google tech you get. The firs day was great. The second and third was a waste of time. The enhanced support got it solved out in about an hour. Nothing more than the gmail SMTP spam filter.

So the moral of the story is, set up a super simple test message. Subject: Test
in the body just “Test”
Anything more like I had, and you are potentially chasing a red herring!

Thank you for sharing this information. I am sure this will help others.

Kind regards.