The spam systems use another trick when they send their spam emails. “That makes sense, but why are they coming to ME instead of going back to the spammer?” Each of those invalid addresses will generate a “mailer-daemon” message back to the spam sender. The problem is many of those addresses may be old, misspelled, or completely made up by the user, so the spam system is unknowingly sending many of its spam messages to email addresses that are not valid. Those are the addresses they will send their spam emails. Spam systems collect any addresses they can from emails they scan, data they mine or buy, and emails they get through websites they control, so they have hundreds or thousands of email addresses in a database. Let’s turn our attention to how spammers operate. “Okay, I get that, but why am I getting them?” That return message will be from “mailer-daemon”. However, if the address does not exist on that server, the message cannot be delivered and must instead be returned to the sender. If the address does exist there, it will deliver the message to that person’s mailbox on the server. When a message is received by a mail server, the server checks the address to see if it exists on that server. So, the “mailer-daemon” (pronounced “male-er day-mun”) is another name for the response system in an email server. Have you ever seen messages from Mail Delivery Subsystem, or “mailer-daemon,” in your inbox? If you have, you have likely seen a LOT of them - from dozens to even hundreds! So what are these messages, why are you getting them, and is there anything to be worried about?

Daemon error message