That already happens. When I ran my own email server, I didn't reject invalid addresses because that was the anti-spam advice at the time (whether or not it worked, I don't know; most of my email was still spam). But that doesn't appear to be what providers like Gmail are doing; they are happy to tell you that an address is invalid. It is somewhat important to human users for the mail server to tell you the address is invalid. (An example: many years ago I interviewed at Google. Some time during the process, my recruiter left. I found this out by sending them an email and getting a message about the address being invalid. I then complained on HN, someone followed up, and I worked there for many years!)