Hi William,
Unfortunately it is not possible to do exactly what you desire

You can do similar things with the request server; it allows you to write a script (for example, "myscript") such that when people send an email to myscript@mydomain.com the script will be run and it can do one thing if the sender is found in a text file and do something else if the sender is not found.
However I can not think of any way to have an auto-responder go through a request-server script automatically. I only mention the Request Server in case in the future you would like to have similar capabilities for emails sent to a specific address, rather than emails auto-generated by the server.