When you send an email to an address that no longer exists, you will receive a response from the mailer daemon indicating that your message was not delivered. If your inbox is suddenly flooded with reports of failed delivery, this may be because someone has been sending emails from your address without your knowledge.

Email works as a virtual mail system. When you send a message, it first goes to a server called the mailer daemon. That server passes the message on to other servers until the message is delivered to the recipient's inbox. If delivery fails, a mailer daemon error message is generated and sent back to the original sender.

Mailer daemons do not use the address in the From line to determine the sender of an email. Instead, a mailer daemon uses the email header, which contains a return path containing the sender's address. By spoofing your address in the email header, spammers can send messages that appear to come from you without having access to your account. If they send an email to an address that no longer exists, you will receive mailer daemon spam.

Because the From line of every email message must contain a sender and spammers don't want to use people's email addresses, they often look up random addresses in someone's contacts to use for phishing and other malicious purposes.