When you get a "Failed Delivery" email from MAILER-DAEMON or a Mail Delivery Subsystem, it means a message you sent was undeliverable, and has been "bounced" back to you.

Bounced messages normally consist of two parts:

• The first part contains the reason for the bounce.
• The second part contains your original message. That’s convenient, because Failed Deliveries usually happen when you mis-typed a person’s email address. If that’s the case, you can simply copy and paste your message from this second part, and re-send it.

Spelling
Double-check the spelling of the address you entered. A single incorrect letter or number will cause a failed delivery.

Incorrect Domain
If the person to whom you’re trying to send an email uses Yahoo! Mail under a domain other than .com (for example, yahoo.au or yahoo.com.sg), ensure that you’ve entered that domain correctly. Email addresses that incorrectly end with @yahoo.com won’t automatically be re-routed to the country you’re hoping to reach.

This error accounts for many failed deliveries.

Their Account
If you’ve verified the address and you continue to receive failed delivery messages, the other person's account could be having problems, or a filter on their end could be blocking your messages.

Over quota
If the second part of a “Failed Delivery” message says,

"This user is over quota"

then your recipient has used up all the space in their email account. Call and ask them to delete some emails so your message can get through! :o)

Message to a Group
If you send a message to a group and you receive a failed delivery notice in return, the message will list the addresses that did not receive your email. You can assume the message was delivered successfully to any of the addresses that don’t appear on this list.

Message Truncated
If you receive a failed delivery notice that states your message was truncated, this refers to the way your message was quoted in the failure notice. It does not indicate that your original message was truncated. If you receive a “Message Truncated” notice, it is not possible to recover the missing text of your original email from the failed delivery notice. Look in your Sent folder instead.

Yahoo! Addresses
If you sent a message to an "@yahoo.com" address and have questions about a failed delivery notice you received in return, please forward a copy of the returned message—including the full header—to us at postmaster@yahoo.com, or to the postmaster@yahoo under whichever domain you’re using Yahoo! Mail—for example, postmaster@yahoo.co.uk.

To view a message’s full header:

• in the all-new Yahoo! Mail, click Standard Header or Compact Header in the upper-right corner of the open message, and select Full Header
• in Yahoo! Mail Classic, click Full Headers in the lower-right corner

Non-Yahoo! Addresses
To receive the most accurate explanation for a failed delivery error from another email service, we suggest you contact their postmaster by sending an email to "postmaster@[their_domain.com]"(for example, postmaster@yahoo.com). Copy and paste the text of the MAILER-DAEMON message to provide them as much information as possible in deciphering the issue.

Note: Do not use "www." when sending messages to email addresses. The "www." is for website URLs, not email addresses.