tl;dr: Is there a way to have VERP probe messages not include copies of the bounce message that triggered them?
Longer form... Some months ago I turned on bounce processing for a fairly large list with a long history, because I could see from the MTA logs that there were easily hundreds of old subscribers whose addresses had become defunct and this seemed like a great way to clean that up and make the server more conscientious at not repeatedly trying to deliver messages to sites that were just going to reject them. This worked really well. Until legitimate subscribers also started to get removed, that is, due primarily to false positives from overzealous spam filters. I had previously read about MM3's VERP probe message feature, which seemed to solve the main reason I had kept bounce processing disabled in the MM2 days, and thought it was on by default but clearly it wasn't. So I found the necessary config option and switched that on. This worked really well. Until legitimate subscribers started to get removed again, that is, due primarily to false positives from overzealous spam filters. See, I realized that the VERP probes were helpfully including copies of the most recent bounce for that user, and those NDRs quite often helpfully included their own copies of the messages which had been bounced. So if a subscriber's MTA was rejecting a message because the content looked like spam, then there was a good chance it would also reject the VERP probe message because it included the same content which had caused the earlier rejection. The VERP probe feature was essentially defeating its own purpose as a neutral confirmation that the subscriber hadn't been rejecting messages due to their content. This behavior raises a more significant concern as well. The MTA on the Mailman server in this case (Exim if it matters) wants to perform batch deliveries when there are multiple subscribers who all share the same remote MTAs. If multiples of those reject a list post, then the NDR back to Mailman aggregates those multiple rejections into a single message. If that incident triggers a VERP probe, the bulk NDR enumerating the addresses of all the subscribers who were rejected gets forwarded along as part of the VERP probe, disclosing a potentially sensitive litany of random subscriber addresses to other subscribers within that batch. I don't know whether this is a misconfiguration, or an unintended consequence of the intersection of Mailman and MTA features. While I understand that the inclusion of a bounce sample within a VERP probe may be a way to take some workload off list admins in that users can possibly see a message/rejection to explain the reason for the probe, the downsides of that choice make the feature unfortunately unusable for some lists that could otherwise have benefited from it. Am I possibly missing an existing switch that could make VERP probe messages *not* include copies of the bounces that triggered them? This is in some ways similar to the problems I've noticed with message hold notifications to moderators including copies of the held messages, since these are (at least in the case of the lists I manage) often spam, so has a tendency to cause the mail providers for the list moderators to start considering the mailing list server a spam source. If there were a switch to make message hold notifications to moderators *not* include copies of the messages themselves, I'd start allowing moderator notifications again. Suggestions welcome! Also willing to try to submit patches to enable some of these solutions if they don't already exist. And thanks to anyone who read this far! -- Jeremy Stanley
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