How can I create a bounce email?

I run my own mail server, which (among other things) handles emails for one of my services. The service in question takes bookings and sends booking confirmation emails. The emails come from noreply@mydomain.com with a Reply To header set to the email address for the organizer.

Some email clients seem not to honor the Reply To header, and so I get emails sent to the noreply@mydomain.com email.

I have setup postfix to catch these and run a script that looks up the event in question and replies to the email detailing the correct email address for the event, and this all works fine.

The problem is that I also get a lot of spam sent to that email address, so I would rather the reply was a bounce (rather than a new email) to discourage spam, but I’m not sure how to achieve this (or if it’s even possible).

What makes a bounce email a bounce email? Does it have to be generated from the sending email server in response to an SMTP error? If so, how do I make it bounce and customize the bounce message (I’m using postfix and this method to run the script). If not, can I modify the email I send in the script to be a bounce email? How do I do this? Is it specific headers in the email?