it doesn’t require it to be “website”@domain. but as you describe shipping@example.com should be the 365 account you are using in the site, signing into 365 and creating an app registration with (be sure it’s not actually signing in as another 365 user).
https://fluentsmtp.com/docs/configure-fluentsmtp-with-microsoft-outlook-office/
Note also at (2) the correct “Supported Account Types” is selected in creating the app registration, and you copy the Value, not the Secret ID, of the Secret.
I recently was presented this error in setup of 365 using FluentSMTP.
The azure app registration had to be made with the email licensed intended email account being used e.g. website@example.com, and not as e.g. the global admin. The FluentSMTP documentation in creating the app though quite good might be possible to improve in this.
