Have you ever received an email with a disclaimer at the end? Something in the line of:
This message is for the designated recipient only and may contain privileged or confidential information. If you have received it in error, please notify the sender immediately and delete the original. Any other use of the email by you is prohibited.
Sounds as a reasonable thing to put, doesn't it?
Now imagine receiving the same email, but with only the sender's name on the TO line. Should I just delete such emails and notify the sender?