Implement core mail list commands and their transport through message header fields

[gadgetto]

RFC 2369 - The Use of URLs as Meta-Syntax for Core Mail List Com
http://www.faqs.org/rfcs/rfc2369.html

3.2. List-Unsubscribe

The List-Unsubscribe field describes the command (preferably using
mail) to directly unsubscribe the user (removing them from the list).
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