We are using Mailman for simple lists. And got this peculiar errot
What does “message has implicit destination” mean? – Documentation – Confluence

2) The message was BCC’ed (blind carbon copied) to the list, and was actually not addressed to the list directly (not that you can see).
If you’re running an umbrella list, you may want this message to be accepted by the sub-list. To do that, in the sub-list’s admin screen, go to Privacy Options / Recipient Filters / Alias Names, and enter the name of the umbrella list as a valid alias.
The reason this happens is that mail being distributed by a mail list retains the name of the mail list in the To: header and the name of the original sender in the From: header. So, when the mail gets to the sub-list, it is still marked as being To the umbrella list. By entering the name of the umbrella list as a valid alias, you tell your sub-list that it’s OK to accept mail which was originally sent to the umbrella list.
Alternatively, if you want to accept all BCCs to the list and get rid of this message entirely, go to Privacy Options / Recipient Filters and set the require_explicit_destination option to be “No”. This should be the first option on the page, and by default, Mailman will set this option to be “Yes”.

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