What does it mean if I have emails from "MAILER-DAEMON"?

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

How can I tell why a message I sent failed to be delivered?

The most common reason for a failed delivery is that the email address entered isn't valid. If the delivery failure message says the account doesn't exist double check the spelling of the address you entered. A single misplaced letter could cause a delivery failure. If the message keeps getting bounced back, make sure the account is closed or hasn't been moved.

Each delivery failure message will provide info on when the original email was sent ("Arrival-Date"), the reason for the failure ("This user doesn't have a aol.com account (XXX123@aol.com)"), and any related error codes ("554 delivery error"). The exact info can be downloaded from the attachment sent in the failed delivery message.

The following is an example of a MAILER-DAEMON attachment:

Image of the text of a MAILER-DAEMON attachment.

What if I can't send emails to a particular address?

Sometimes you'll get an error message "550 is not accepting mail from this sender." This often means your email address has been blocked from sending mail to a specific contact because your address has been blocked by privacy or spam control settings set by the owner of that account.

Should this happen, you'll need to check with that contact to make sure you haven't been accidentally added to their blocked or spam list. Depending on who their email provider is, these settings could be in various places in their account.