I can absolutely see how that message would be concerning, luckily that is not what your recipients should be seeing. Spoofing is essentially when someone 'pretends' to be you while sending an email. It looks like that email you tested came 'from' the same address you were sending to. Your inbox simply recognized that you didn't actually send that to yourself, and is warning you that someone else is pretending to be you. If you send those tests to other addresses, they should not receive the same warnings.