There are many reasons this can happen, but e-mail "bounces" back to the
sender usually under 3 conditions:
1. The domain of the recipient's e-mail address does not exist
2. The e-mail address of the recipient no longer exists
3. The SMTP server of the recipient's domain was not reachable for a
long period of time.
By "bouncing", your original e-mail message comes back into your Inbox
along with a standard error message produced by the SMTP server. This
error message should give you a good indication of the problem.
Read through the details of the bounce back message to find out exactly
why it was returned. The message will also inform you if it is a
temporary failure or a permenant one. Sometimes you may receive a
temporary failure, but never receive a permenant one meaning it wasn't
able to be delivered immidiately, but did eventually go through. Most
servers are configured to permenantly bounce the message if it still
fails after 4 days.