There are many reasons this can happen, but e-mail “bouncing” back to the sender is usually caused by one of three conditions:

• The domain of the recipient’s e-mail address does not exist.
  


• The e-mail address of the recipient no longer exists.
  


• The SMTP server used by the recipient’s domain was not reachable for a long period of time.

By “bouncing”, your original e-mail message is routed back to your Inbox with a standard error message producted by the SMTP server. This error message should give you a good indication of the problem.