Text from mailer daemon

The infamous Mailer Daemon!

The Mailer Daemon is a system that sends error messages to the sender of an email when there's a problem with the email delivery. These messages are usually generated by the mail transfer agent (MTA) or mail delivery agent (MDA) on the receiving end.

Here are some common examples of text from the Mailer Daemon:

Undeliverable email

• "Delivery Status Notification (Failure)"
• "Undeliverable: [email address]"
• "The email address is not valid"
• "The recipient is not available"

Temporary failure

• "Delivery Status Notification (Delayed)"
• "Temporary failure. Message will be retried later."
• "The message will be resent when the recipient is available"

Permanent failure

• "Delivery Status Notification (Failure)"
• "Permanent failure. Message will not be resent."
• "The recipient's email address is invalid or does not exist"

Other errors

• "Connection refused"
• "Authentication failed"
• "Invalid sender email address"
• "Message too large"

These error messages usually contain information about the problem, such as the recipient's email address, the error code, and a brief description of the issue. They're designed to help the sender troubleshoot and resolve the problem, so they can try again to deliver the email.