plain text e-mails are send without line-breaks

See if setting $message['headers']['Content-Type'] to 'text/plain' helps.

I figured out a bit:

postmark/src/Plugin/Mail/PostmarkMail.php::118

    if (isset($message['plain'])) {
      $postmark_message['text'] = $message['plain'];
    }
    else {
      $converter = new Html2Text($message['body']);
      $postmark_message['text'] = $converter->getText();
    }

If the $message already is formatted as plain/text,
this Html2Text will effectively destroy any "\n" line breaks.

I dont see how $message['plain'] is filled anywhere, except doing it manually in a hook_mail_alter()
(This is our Quickfix for now since we only need to send plain/text mails)

Also in the same class inside the format function at ::89

$format = $this->config->get('format_filter');
    if (!empty($format)) {
      $message['body'] = check_markup($message['body'], $format, $message['langcode']);
    }

There is no format_filter inside the postmark.settings.yml nor is there a Interface for it.

I guess this is a good place to check if this email is html or plain/text and set the
$message['body'] and $message['plain'] accordingly. Either using that format_filter config setting
or using the $message['headers']['Content-Type']?
Question is where it is set anyway..

Also setting anything inside $message['headers'] are not passed along to the postmark client library.

Maybe we should add an alteration hook to the module. In this case, we can control the $postmark_message variable / mail created for Postmark service.