There are several possible causes for either a mail delivery failure notice, or a mail delay notice:

If there is a problem at either the sending or receiving domain, mail delivery will fail or be delayed.
This will result in a mail failure/delay notification. This mail failure/delay notification let's you know
why your message was not sent or got delayed.

To obtain more information on how to understand a mail failure/delay notification, please click here

Sometimes you will receive a mail delivery failure notice for messages you have not sent. In
almost all such circumstances the message sent was spam/virus/email fraud. This situation
happens when your computer is infected by a virus that sends emails using your email address in
the ' From' and/or the ' Return-Path' fields.

Spammers, hackers, and many viruses often forge email addresses into the ' From' and/or the
' Return-Path' fields of original message headers in order to disguise the actual origin of the
message. In almost all cases of forgery, the message is sent from a different location and not
from your email account and/or email service. Also, spammers and viruses send these messages
to random email addresses in the To:, Cc:, or Bcc: fields. If the recipient email address happens
to be valid, they' ll receive the spam/virus/email fraud. However, if the recipient email address is
invalid, you' ll receive a mail delivery failure notification

To obtain more information on spoofing/forgery, please click here.