Mailer Daemon is an email verification protocol that is set in place to make sure that email gets from one place to another. If the email that has been sent has a problem along the way, the sender may receive a message from Mailer Daemon. There are multiple reasons why an error could occur with an email, but rest assured, if an email does not reach its destination, the Mailer Daemon will let you know about it.
The most common cause of a Mailer Daemon message is that the sender has misspelled the recipient’s email address. Sometimes in a matter of seconds and typically within 24 hours, if you have misspelled an email address, the server you are trying to contact will send you a message telling you that the e-mail address you entered was incorrect.
While this doesn’t happen as frequently anymore, you may receive a Mailer Daemon message if your recipient’s mailbox is full. If there is no space left in your recipient’s account, the server will attempt to deliver the message and fail. This will generate the Mailer Daemon message to be sent back in your direction.
Detected as Spam
Due to the prevalence of spam (abusive bulk messages) in email, some servers may block messages. This means that if you send an email from a blacklisted server, your message may be sent back by the Mailer Daemon.
Problems at the Source
If the server you are trying to send a message to is unavailable, you may receive a Mailer Daemon message. If you send an email to a server, but the server is down for maintenance, you will receive a message from Mailer Daemon notifying you that the message was undeliverable.