Steve Bachinsky writes:
The lists are configured for “require_explicit_destination”. However, the email address of the list is clearly in the CC: header of the email (no alias or subdomain). Is there some other reason why Mailman would hold an email for moderation with the reason of “implicit destination”?
Since as Mark explained there's no such reason, and there's no obvious "common error" that would explain it, I'm going to list a couple of "wild guesses". It would help if you would show us exactly what you are seeing.
I would guess you're looking at the "raw" or "source" version of the message. But if you're looking at it in a "modern" "helpful" MUA (or it was sent by such), it's possible that according to the header parser the "address" you're looking at is actually the display name (often but not always enclosed in double quotes ""), not the address (usually but not always enclosed in angle brackets <>).
Perhaps there is a missing comma, so that one of the addresses looks like "problem-list@your-host.com <some-address@somewhere.on.earth>" and there should be a comma after "com". How it's getting to the list that way I don't know; perhaps the list is subscribed to another list (see next point).
Are you sure that the moderating list is the same list that is in the CC? In the common configuration where a high-traffic discussion list is subscribed to a low-traffic announcement list, announcements will be held by the discussion list unless the announcement list is configured as an alias, or the discussion list allows implicit destination.
I can think of a couple of possibilities involving internationalized email addresses (eg, a-labels vs. u-labels which Mailman knows nothing about yet, but should be treated as equivalents). But this seems *really* unlikely, even the IETF has not figured out how to do EAI with mailing lists yet, and almost nobody uses it at all yet.
This moderation is not consistent for same list and poster. The emails come from a large domain with multiple load-balancing email servers. I suspect that it may be something with one or more of the servers being misconfigured, but unable to see anything obvious from the headers and confused why Mailman states the reason to be implicit destination when that does not seem to be the cause.
I haven't thought carefully about it, but the only such possibility I can think of is that the upstream server is deleting the comma or ignoring the fact that there should be a comma. Otherwise, whatever is happening is local to your MTA + Mailman system, I'm pretty sure.
I hope one of the wild guesses helps you figure out what's going on, or helps you find a more useful anomoly.
-- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan