This is an implementation of https://code.launchpad.net/~jimpop/mailman/check_subscriber for Mailman 3.
A Postfix milter checks whether an email address is a subscriber to a mailing list. If it is not, it rejects the mail during the SMTP transaction.
This avoids sending backscatter-bounces, and avoids blackholing messages from non-subscribers — the sender's MTA will generate an error.
In master.cf:
127.0.0.1:26005 inet n n n - 1 spawn