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.