Mailer-Daemon email message

The Mailer-Daemon message is from an automated program which returns email sent over the internet back to the sender if the email is undeliverable.

Email may be undeliverable if the recipient's mailbox is full, their email server is experiencing difficulty, or the email address is incorrect.

Mailer-Daemon example
Your mail may be returned due to one or more non-delivery conditions. When a mail server returns email as undeliverable, the domain's postmaster normally generates an automatic response. The actual error is indicated in the 'Transcript of session follows' section of the return reply.

There should be a message that contains something similar to the following:

----- Transcript of session follows -----
... while talking to air29.mail.aol.com.:
>>> RCPT To:<(email address)@aol.com>
<<< 550 Mailbox not found
550 <(email address)@aol.com>... User unknown

Most common cause of a Mailer-Daemon
The most common cause for receiving a Mailer-Daemon message is an incorrectly addressed email. An internet email address must be typed correctly with no spaces and with the @ symbol in the correct place, for example, user_name@domain.name.com.

Unknown host
The domain.name.com section of the internet email address may contain spelling errors. It is also possible that the computers located at that internet address are temporarily unavailable.

Unknown user
The user_name section of the internet email address may contain spelling errors.

Service unavailable
The receiving site is down temporarily.

Unbalanced
The address may contain extraneous quotation marks or parentheses.

Cannot send for XX days
The AOL service is having trouble communicating with the receiving site so the email was not delivered.

Unrecoverable Error
The email left the AOL service properly, but critical errors occurred within the receiving system.