What version are you running?
3.0.14
What's the URL of the page this enhancement relates to, if any?
emailing
Describe the enhancement and the motivation for it.
Gmail apparently uses message-id as one of the fields it uses to "thread" a conversation.  When using an AWS SES SMTP endpoint as the configured email server the emails appear as individual messages in Gmail rather than a threaded conversation.  So if there are 100 reviews/comments, there are 100 corresponding messages in Gmail rather than a single thread.
What operating system are you using? What browser?
Ubuntu / Chrome / Gmail
Please provide any additional information below.
If RB could grab the SES message-id from the result of the smtp session instead of the internally generated message-id, then Gmail would be able to correctly thread messages.
These are examples of replaced message-id's in messages delivered via SES:
Message-ID: 02010178ffc92fd9-f89a51ee-deb7-41af-aca1-dcf6df27bcea-000000@email.amazonses.com
Message-ID: 02000178ffca483d-0148843b-2bf9-482f-a654-bc6d9d620155-000000@email.amazonses.com 
This is the SES endpoint we're using:
email-smtp.us-east-1.amazonaws.com
But they have many more in different AZs, etc...