Also posted this question to the mailman3 list yesterday. Andrew Hodgson sent an informative answer, so that's probably it. "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 9:18 AM Sam Darwin <samuel.d.darwin@gmail.com> wrote:

Hi,

We are running mailman2 with postfix and CentOS. For a long time the mail gateway was mailgun. Recently switched to Amazon SES.

Now there are complaints that "threading" isn't right. That is, organizing the results according to topic, and correlating replies with specific other replies.

It turns out that "The Amazon Simple Email Service (AWS SES) rewrites the Message-ID header of outgoing emails."

This means the References and In-Reply-To headers can't be relied on anymore also, since they won't always match up. These headers:

Message-ID: <127401d9389b$e874b7f0$b95e27d0$@gmail.com> References: <126301d93891$58081b80$08185280$@gmail.com> In-Reply-To: <126301d93891$58081b80$08185280$@gmail.com>

might now appear as:

In-Reply-To: <010f0187dca41ec8-478dfff5-d27d-4879-ab72-072be2f76176-000000@us-east-2.amazonses.com> References: <010f0187d80eb0dd-93bebdd9-cabf-4ad6-a7f4-b5515bd22d31-000000@us-east-2.amazonses.com> <010f0187dca41ec8-478dfff5-d27d-4879-ab72-072be2f76176-000000@us-east-2.amazonses.com>

But perhaps a main problem is that for a large outgoing mailing, the Message-ID might not be consistent when sending to 1000's of recipients. Maybe not everyone sees the same Message-ID.

Any thoughts on the topic?

Has anyone used Amazon SES, and does the "threading" in the archives line up correctly?

Thanks, Sam