Currently looking at the mail plugin GovUKNotifyMail::mail() it expects the $message['body'] to be an array, however, it does seem that the MailInterface::mail() expects it to be an already formatted string.
It expects that the message body has already been formatted to a string by the time it reaches the plugin's mail() method. It appears that we are not handling this as expected in the format() method.
Instantiate a mail plugin but send the message later:
$message = \Drupal::service('plugin.manager.mail')->mail('example', 'notice', $to, $langcode, $params, FALSE);
$message['body'] = "An externally formatted mail body."
$system = \Drupal::service('plugin.manager.mail')->getInstance([
'module' => 'example',
'key' => 'notice',
]);
$system->mail($message);
Needs work
2.1
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Thanks @kalpaitch for the patch and @i-trokhanenko for confirming it works.
Can we add a test for this?