LibAnswers

This is a known email behavior, not a problem with the ELUNA mailing list system.

Short answer

When a message is sent to more than one mailing list at the same time, both copies go out with the exact same Message-ID (an internal tracking header, invisible in your inbox). If a recipient is subscribed to more than one of the lists you sent to, their email system treats the second copy as a duplicate and silently drops it. This happens on the recipient's side, most commonly in Microsoft Exchange/365 and Gmail, and there is no setting on the ELUNA list server that can prevent it.

Why this happens

Mailing list software (including the GNU Mailman software ELUNA uses) intentionally keeps the original Message-ID unchanged when it redistributes a message to a list. This is correct behavior according to email standards, since changing that header would break reply threading in most email clients. The tradeoff is that if the same message reaches one mailbox twice, by way of two different lists, mail systems that perform duplicate suppression will only deliver the first copy.

Who this affects

Only recipients who are subscribed to more than one of the lists a message was cross-posted to. If someone is on just one of the lists, they should receive it normally.

Recommended workaround

If you need to reach multiple ELUNA lists with the same message, send it as separate individual emails to each list rather than one email addressed to multiple lists at once. This avoids the duplicate Message-ID entirely and ensures everyone receives it.

Further reading

This is a long-documented interaction between mailing list software and duplicate suppression features in modern email systems. For a technical discussion from GNU Mailman's own developers, see: https://mailman-users.python.narkive.com/NH7oUl6r/mailman-reusing-message-id-leads-to-duplicate-message-suppression

Last updated: September 2026