Getting these “mailer-daemon” messages means the spam email has already been sent. Servers vary in the length of time for which they continue to try delivering the message before returning it to the sender, so you may receive these “mailer-daemon” messages for up to four days before they stop.

Is mailer-daemon a virus?

If you open an email containing a virus or worm, it can infect your computer and send infected messages to everyone in your address book. Receiving mailer-daemon spam doesn't necessarily mean you have malware, but there are some precautions you need to take.

What does it mean when you get mailer-daemon?

When you get mail from a "MAILER-DAEMON" or a "Mail Delivery Subsystem" with a subject like "Failed Delivery" or similar, it means that an email you sent was undeliverable and has been "bounced" back to you. These messages are produced automatically and usually include a reason for the delivery failure.