In retrospect it was an irresistible experiment to try out Amazon SES. The service seems to offer a large cost savings while also providing reliable mail delivery. However, here is a reply from the mailman-users list: "Amazon SES rewrites the Message-ID. Yep and for this reason I don't use it. The problem is worse in Mailman3 because it has a much more superior archive display which gets ruined when people reply to messages with different Message-Id headers and the threading can't be observed. I also had to use the anonymous mode with my list as Amazon didn't like me using random addresses in the From: header. I'm still looking for a tolerant mail gateway to use with Mailman." On Thu, May 4, 2023 at 5:27 PM Gavin Lambert via Boost wrote:

Mere moments ago, quoth I:

...

There's not too much problem if the first message id is rewritten (provided both mailman and SES agree on what it is, but that seems hard if mailman is earlier in the traffic chain, as appears to be the case).

Actually, I take that back. Messages might be cross-posted to multiple lists, in which case it's important that these all have the same message id too. While *usually* they would all be Boost lists, that's not necessarily the case, so the only correct choice is to not alter the original message id.

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