The bounce command was removed in Lion.

Here is how to add the functionality back using Automator.

1. Open Automator.

2. Create a new service.

3. Configure the service so that it has "no input" in "Mail"

4. Drag "Get Selected Mail Messages" into the workflow

5. Drag "Run Applescript" into the workflow

6. Use the following AppleScript, then save the workflow with a name like "Bounce Message".

on run {input, parameters}

tell application "Mail"

repeat with eachMessage in input

bounce eachMessage

end repeat

end tell

end run

7. In Mail, select the message you would like to bounce. Then from the "Mail" menu, choose "Services", then click on your new service. The message will bounce.