Chances are that sooner of later you're going to get a message with that heading in your Inbox telling you
that a message you sent failed to reach its destination. At first glance the contents of the error message may
look like nonsense, but you can look for these clues to give you insight into what went wrong:
Unknown host (or Host unknown): This is probably the most common error message that you'll receive. It
means that the mail delivery system can't find the address that you gave it. Double-check the e-mail message
that you sent and make sure that you didn't mistyped the address.
Network Unreachable, Connection Timed Out, or Bad File Number: A message like this usually signals that the
recipient's server is down (or experiencing some other kind of problem). The only thing you can do is to wait
and try again later. This kind of problem is usually temporary, but you may have to wait anywhere from a few
minutes to a couple of days before the message is delivered successfully.