Mail.app: How to Bounce a Message to the Sender
Contributed by Rob Rogers 
Tagged: Mac OS X Leopard If you receive an email from someone that you really wish didn’t have your email address, you can make them think that they don’t. By using the Bounce functionality that is included in the OS X mail client to fool the pest into thinking that their email did not reach the intended goal. The sender will receive a notification that the email was not delivered because the addresses had permanent fatal errors and the user is unknown.
To Bounce a message from the Inbox:
1. Right-click the desired message.
2. Select Bounce from the menu.
The message will disappear from the Inbox.
To Bounce a message from the Message Window:
1. If you are viewing the trouble email, go to the Menu and select Message
2. Select Bounce.
You will return to the Inbox, where you will find that the message is no longer there.
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Miztee said on April 11, 2011
Why does the offending message go to my trash mailbox after the bounce? And I’m still getting emails from this person after bouncing for weeks. Aargh!