Why would you want to autoreply to spam ? there are sevreal reasons NOT to do this.. And I really can't think of any GOOD resons to respond to it..
1. replying to a SPAM Message may verify your emai laddress to the Spammers so you start getting MORE SPAM
2. The From addressess on Most SPAM messages are invalid so replying to these messages will fill up your queue, The messages will sit in your mail queue until they finally time out and are dropped.
3. if you recieve 50,000 spam messages a day and reply to them all, you are doubling the load the SPAM puts on your mail server. Why would you want to double something that is bad to begin with ?
Do yourself a favor and mark or throw away spam, NEVER waste your time or computing resources responding to it.