Improve Deliverability: add list-subscribe-header

[funktionierbar]

Hi, I tested one of the Mails going through our mailing list with the tool mail-tester.com. It suggested to add the header "list-unsubscribe) to improve deliverability. As this is a major concern with my setup (selfhosted mailserver) I would suggest adding this header to the mails. More info can be found here. https://www.sendinblue.com/blog/list-unsubscribe-header/

I am sorry not to be able to contribute. Thank you for your work!