This Trac plugin will add header's Message-ID field to References field.
Amazon SES changes header's Message-ID to its own format, which causes mail clients unable to identify the relation of threading. (Mails from second mail have References field.)
This plugin implements an workaround. It will copy Message-ID to References if no References field existed, therefore rebuild relationship between first mail and followings.
Only tested on Trac 1.4, but may work on Trac 1.2 since it uses IEmailDecorator only.
pip install git+https://github.com/gslin/trac-references-mail-decorator.git
Then restart Trac and enable it from the admin interface.
https://github.com/weisslj/trac-messageid-plugin is also trying to solve this issue, with different approach.
See LICENSE file.