SMTP relay service error messages - Google
https://support.google.com/a/answer/6140680The IP address you registered in the Google Workspace SMTP relay service setting doesn’t match the domain of the account this email is being sent from. If you are trying to relay mail from a domain that isn’t registered in your Google Workspace account, you must configure your mail server either to use SMTP AUTH to identify the sending domain or to present one of your …