Thanks Alex, I guess I was not clear. I have installed the AppleScript from this thread, making it a Service via Automator, but it is not working for the particular application I need, which is for the bounce message to make clear which address was undeliverable, not just making it appear that my address was undeliverable. This is becuse my mailbox is the so-called "catch-all" or "default" for my domain.
In other words, if my address is abc@mydomain.com, and someone sends to def@mydomain.com, I will get that message due to the way the domain is configured. (This is extremely common for personal domains.) However, if I use the bounce script, the NDN that is sent will say:
abc@mydomain.com: no such user
when I want it to say:
def@mydomain.com: no such user
It would be nice to be able to configure the bounce message and have it use the "To:" line of the message instead of my address, but I don't know where the code for the AppleScript "bounce" command refrerenced in the script is and how, or even if, it can be modified.