Email Header Improvements and Suggesstions

[VBGAMER45]

Was working on a mod to add
List-Unsubscribe
List-Unsubscribe-Post: List-Unsubscribe=One-Click

Due google new change which makes it urgents: https://support.google.com/mail/answer/81126?sjid=5308947540389480254-NC which affect people sending more than 5k a day to gmail.com This would affect newsletter mailings and large sites.
I think this should be looked at for SMF too those headers.

Noticed the headers Message-ID not unique per email sent same along with date/time

$headers .= 'Date: ' . gmdate('D, d M Y H:i:s') . ' -0000' . $line_break;

if ($message_id !== null && empty($modSettings['mail_no_message_id']))
$headers .= 'Message-ID: <' . md5($scripturl . microtime()) . '-' . $message_id . strstr(empty($modSettings['mail_from']) ? $webmaster_email : $modSettings['mail_from'], '@') . '>' . $line_break;

Suggestion:
Sender header added.
$headers .= 'Sender: ' . (empty($modSettings['mail_from']) ? $webmaster_email : $modSettings['mail_from']) . $line_break;

For @gmail.com add Feedback-ID: header acts as a FBL.
Feedback-ID:
https://support.google.com/mail/answer/6254652?hl=en

[jdarwood007]

I have a PR for message id fixes: #7843
That should make the message ID compliant

[VBGAMER45]

Great. I am thinking it should be probably unique though for each email going out. Was working on a mod thought about using hooks then realized i couldn't set custom headers for each email sent out, since values change., nor see the email address $to

[Sesquipedalian]

Moved to Alpha 5, as per the internal roadmap.