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.