Good Day! So from the error message itself you can see the google support link generated to troubleshoot, try all the steps in Step 2, like may be turn off 2FA if not already, you have turned on Less secure app access, then try the unlock captcha one. Hope this helps.
It looks like in the red error message you have to sign into the Google account that is trying to send the message.
In the past I have seen this where you need to manually log in and complete a captcha first before you can use the automation.
To use Gmail you have two options. If 2FA is enabled, you need to generate an app password via https://security.google.com/ or https://myaccount.google.com/ and use that to login in the Mendix app. If at any time the access has to be revoked, you can simple revoke it on the same page as where it was generated.
If 2FA is not enabled, you can turn on “less secure app access”. This will let you use the account password to log in. Of course, this means the full account access password is saved in the database. If someone gets access to your account this way, you'd have to change the password.