Formatting of the digest email

it's happens because module set email header = text/html
By default Drupal use email header text/plain.

If the module wants to support text/html that's fine, but I agree with @Chilic, text/plain should at least be the default. Does this need to be set explicitly by message_digest? Can we expose it as an admin option?

Similar issue with header at Drupal Association site. Maybe related.

Would it be possible to do it like this?

<?php
/**
 * Implements hook_mail()
 */
function MYMODULE_mail($key, &$message, $params) {
  if ($key == 'digest') {
    // Remove the HTML formatting.
    if (isset($message['headers']['Content-Type']) {
      unset($message['headers']['Content-Type']);
    }
  }
}
?>

If you install the Mime Mail module then it works fine - https://www.drupal.org/project/mimemail (Mime Mail requires Mail System to be installed so you'll need that too - https://www.drupal.org/project/mailsystem)

Install both, enable, and you should get HTML emails sent fine without any further config.

Thanks @ValiH for the tip. Will try!

I can't see a reason for hard coding 'text/html' in message_digest_mail().
Patch #1 should be committed.

Patch #1 didn't apply any more.
I've re-rolled it.

Thank you @PawelR.

RBTC +1

I'm using Mime Mail and Message digest isn't picking up my mail.css styling, although message_notify is. ?

The patch in #8 worked for me. Thanks!

Corresponding patch for 8.x. I think the 7.x version can also be committed.

Committed to 7 and 8. Thanks!