No idea.
My first thought would be that sending an email to every possible random string would be a quick way to get flagged by whatever big city backend blacklisting goes on. If I were trying to set up my own email server for my own domain, unless there's some rule that says I have to, I'm not sending a no-such-address message back. Making a success and a failure look different to the attacker (which I'm going to treat spam senders as for the purposes of histrionics) is giving them a signal for free.