Implement feedback loop support FBL for gmail

[brendanheywood]

https://messagebird.com/blog/all-you-need-to-know-about-gmail-feedback-loop-offering

https://support.google.com/a/answer/6254652?hl=en

Feedback-ID: CampaignIDX:CustomerID2:MailTypeID3:SenderId

• CampaignIDX - frankstyle component
• CustomerID2 - user id
• MailTypeID3 - maybe message / notification type?
• SenderId - should be aligned to the moodle site

Feedback-ID: mod_forum:user123:post:moodle.org

https://senders.yahooinc.com/complaint-feedback-loop/

http://postmaster.mail.ru/

https://support.msn.com/eform.aspx?productKey=edfsjmrpp&page=support_home_options_form_byemail&ct=eformts&scrx=1

Depending on the provider this may give us insight into reputation rates across many sites but broken down by site