Checking original messages locally in my inbox messages, it appears mailgun doesn't rewrite the ID. SES rewrites the ID. Not an option to disable it. I will open a discussion with mailman and see if they have any suggestions. On Thu, May 4, 2023 at 10:30 AM Andrey Semashev via Boost wrote:

On 5/4/23 18:54, Sam Darwin via Boost wrote:

...

The comments about Message-ID: , References: , and In-Reply-To: headers are very interesting. "The Amazon Simple Email Service (AWS SES) rewrites the Message-ID header of outgoing emails." Consider the very first email of a thread. It arrives at the mailman server with a Message-ID that mentions "gmail", for example, and is added to the Archives. The message is sent out to all list members, who see a new Message-id of ...010f0187dca41ec8@us-east-2.amazonses.com Later replies can set the In-Reply-To header to refer the "@us-east-2.amazonses.com" Message-ID. But the first archived message didn't yet have that ID, since it arrived directly at the server, not through SES. The mailman archives on lists.boost.org organize that first message in a separate thread, presumably based on the original Message-ID.

Can we disable Message-ID rewriting?

Do we know how mailgun works wrt. Message-ID and other headers?

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