Have you ever received messages from the Mail Delivery system, or “mailer-daemon,” in your inbox? If you’ve, you’ve possibly seen plenty of them. So, what are these messages, and why are you receiving them? It is not personal – simply a computer. Once a message is received by a mail server, the server checks the address to determine if it exists in this server. If the address exists there, it’s going to deliver the message to the person’s mailbox on the server. However, if the address does not exist on this server, the message cannot be delivered and should instead come back to the sender. That come back message is from “mailer-daemon.”