I forgot to include the mailing list in a reply to Stephen and I wantied to follow up here in case what I learned is helpful to others.

One of the things Stephen suggested was that postfix was filtering the headers. That was true in my case. The configuration code I inherited had the following lines in /etc/postfix/header_checks /^List-Unsubscribe:/ IGNORE /^List-Subscribe:/ IGNORE

How are you capturing mail at postfix? For capturing the email at postifx I used the following steps I found on another website to defer SMTP sending so I could look at emails while they were "in process" by postfix.

postconf -e defer_transports=smtp; postfix reload # send the email to the server mailq # Use ID from mailq to examine the email using postcat postcat -vq C23D82B60024 # Re-enable SMTP sending postconf -e defer_transports=; postfix reload; postfix flush

As for the question about debugging in rfc-2369.py, I did have additional log.debug calls in the "process" method, but was not seeing anything from them. Perhaps they were not getting called as Stephen suggested.

Stephen also mentioned in a private email to me about RFC 8058 "One-Click Leave" which is intended to make it easier for folks to unsubscribe from lists. We don't have an implementation in Mailman 3 yet, but it looks like a reasonable protocol (ie, not that easy to abuse, though not impossible). It can probably be implemented in Postfix (but it might not be straightforward). https://datatracker.ietf.org/doc/html/rfc8058

The contents of this e-mail, including attachments, are intended for use by the named addressee only and may contain information that is confidential or private according to applicable law. If you received this electronic transmission in error, please notify the sender by a reply to sender only message, permanently delete the original message and any attachments from your electronic device and keep this message confidential. Unauthorized use, copying, review and/or distribution to unauthorized persons is strictly prohibited and subjects the user to penalty of law. Thank you for your cooperation.