Is it possible to trigger a 550 SMTP answer from a script Postfix piped a mail to?

I want to prevent backscatter from within a mailing list script that I use on my Postfix mail server.

Mail for a mailing list hosted there is piped to my script. At this point, Postfix can't know if the sender address is subscribed to the list, but in each case, the mail is accepted and delivered, if the list address is existing.

Now if the sender isn't subscribed, the list generates an according answer and sends it to where the original mail came from. This is of course meaningful in case some subscriber e.g. sent to the list using the wrong address, but it may also cause some spam to be answered or even a forged sender address receive the answer of the list.

If I receive mail for non-existing addresses, the server answers with 550, and the other side has to care about informing the local user or whatever. I'd like to trigger this behavior also for the case that I receive mails for a list the sender isn't subscribed to. But if I let my script exit with a non-0-status, Postfix will send some error message, which would be the same backscatter for spam mails as if the list itself would send a "not subscribed" mail.

So is it possible, from within a script mail is piped to, to tell Postfix to reject a mail at SMTP level? So that my server doesn't send anything (and a properly configured server will still inform a possibly legitimate user)?