Cannot access message-id header

Problem/Motivation

It would be nice to have emails appear as threaded in email client. At the moment there seems to be no way to get message-id from email after it has been sent.

If you extend EmailBuilderBase, implement postSend and print out the parameters like this:

$email->getHeaders()->toArray()

you get

Array
(
    [0] => X-Mailer: Drupal
    [1] => Subject: something
    [2] => Sender: "some-domain" 
    [3] => To: "sender-email-address" 
)