What's stopping spammers from just having a GMail account they can use to test if the message made it through? I mean; I have to assume the engineers in charge

What's stopping spammers from just having a GMail account they can use to test if the message made it through? I mean; I have to assume the engineers in charge of this @ Google are making the right call, but the cynic in me can't help but worry they are making the call in a way that benefits them at the cost of a healthier SMTP ecosystem. (ie, "Lets make email just unreliable enough that folks switch to Google Email 100%, or use some other Google Product for messaging ...") You can reject the message anytime after TO/FROM/DATA w/o signaling which of those 3 caused it the message to be rejected.