Initial notification email is not being threaded together with replies when using Amazon SES
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@jindal1 reported in gitlab-foss#60081 (comment 284184861) that initial notification emails (issue / MR creation notifications) are not threaded together with the other notifications for that issue / MR.
When we send these issue / MR creation notifications, they are not treated as replies and we do not set the In-Reply-To or References header. We just set the Message-ID to something like <merge_request_xxxxx@gitlab.com>. This is because since this is about the creation of the issue / MR, it shouldn't be a reply to some other notification.
On the subsequent notifications about that issue / MR, we then set the In-Reply-To and References to reference that initial Message-ID.
From the discussions in that thread, this may be an issue with Amazon SES because it is changing the Message-ID header (merge_request_xxxxx@gitlab.com)
cc @toon