If I recall my time at a main-sleaze email sender, some ISPs will send SMTP retry requests on random incoming email to see if there is a functioning SMTP server

If I recall my time at a main-sleaze email sender, some ISPs will send SMTP retry requests on random incoming email to see if there is a functioning SMTP server on the other side of the originator's email. Often times, spammers won't care about the email that's being sent, so won't respond, or in cases where they are hiding the origin, won't be able to respond.