There are many reasons this can happen, but e-mail "bounces" back to the sender usually under 3 conditions:
- The domain of the recipient's e-mail address does not exist
- The e-mail address of the recipient no longer exists
- The mail server of the recipient's domain was not reachable for a long period of time.
- The mail server of the recipient's domain thinks your email is spam or has unwanted attachments.
By "bouncing", your original e-mail message comes back into your Inbox along with a standard error message produced by the mail server. This error message should give you a good indication of the problem.