The Amazon Simple Email Service (AWS SES) rewrites the Message-ID header of outgoing emails. That means subsequent failure/success notifications will not be in the same thread, because they reference a non-existing message id in the In-Reply-To header.

The rewritten message id is returned as last message of the SMTP transaction, e.g.

250 Ok <00000123456abcde-1234abcd-abcd-1234-1234-1234abcd1234-000000@eu-west-1.amazonses.com>

http://docs.aws.amazon.com/ses/latest/DeveloperGuide/smtp-response-codes.html

I have written a proof-of-concept patch that updates the rewritten message id in case AWS SES is used for sending mails.

Would you be willing to accept such a patch? It is kind of ugly to include provider-specific code like this, but it has no downside for people not using AWS SES and can probably be encapsulated so that it doesn't get in the way.