Reference headers still being butchered. Your message id was:
Message-ID: <0000014aac80139f-0a0965b9-fad8-4aef-bedb-b381a6bcf84f-000...@email.amazonses.com>
But Chuck's reply had:References: <0000014aac801767-9a0a20cc-a742-4e06-b342-7a54b23c22f4-000...@email.amazonses.com> In-Reply-To: <0000014aac801767-9a0a20cc-a742-4e06-b342-7a54b23c22f4-000...@email.amazonses.com>
So it still didn't thread properly. The In-Reply-To should have matched the Message-ID.
~Seth