[joahua]

I get this on my domain, it’s called backscatter I think. https://en.m.wikipedia.org/wiki/Backscatter_(email)

Less technical folks would likely complain about it, as it tends to resemble the spam that was sent from their spoofed domain (I.e. “re: fake pills” subject lines as the rejection message hits their inbox).

[willcipriano]

In my case this isn't it. What happens is during the connection process the server on the other side first announces what email its sending from, at that point my server responds with a reject message and closes the connection. I don't think my server even sees the subject or body. My server doesn't send a email, but instead that email is generated by the sending server to let the user know the message was rejected.

This error message is often delivered to a catchall account, they are often configured with small mailboxes, that box fills up, throws alerts and ticks off a sysadmin somewhere. I only do this in cases where it's easier to SSH into my box and configure it than unsubscribe, and never to random address spam as that is wack-a-mole and futile.

I think the sending servers may also be misconfigured to get around greylisting. As sometimes they will try repeatedly to deliver to me. My server dutifully rejects each attempt and the messages pile up on the other side.