How to add a custom header MessageStream: broadcast for Postmark?

You can do $email->getHeaders()->addHeader()

Thanks for the answer. I found the Hooks → page, and managed to get it working:

<?php
use Drupal\symfony_mailer\EmailInterface;

/**
 * Implements hook_mailer_TYPE_build() for emails of type XXX.
 */
function my_custom_mailer_simplenews_build(EmailInterface $email) {
  $email->getHeaders()->addHeader("MessageStream", "broadcast");
}

I added a "Add custom headers" section to the Hooks → documentation page.

I wasn't initially sure how to find the value of "emails of type XXX" mentioned, but managed to find the src/Plugin/EmailBuilder folder, which had some email types. I guess for CommerceOrderEmailBuilder.php, you would need to replace XXX with commerceorder?

My task is solved now, but I do still think that adding support for custom headers would be a nice feature, especially for those users who don't have the know how to build a custom module ...

Automatically closed - issue fixed for 2 weeks with no activity.