Allow me to distill this very insightful article from Word to the Wise down to four simple points:
- Microsoft wants you to include the list-unsubscribe header. And today, you'll want to use the "mailto" version, not the "http" version.
- If you don't, Microsoft is going to make it very easy for Hotmail/Outlook.com subscribers to BLOCK your mail, when in fact they perhaps only wanted to unsubscribe.
- If subscribers BLOCK your mail, they're not going to get any followup transactional mail, which isn't great. Or if they opt-in again later, they won't receive that new mail.
- It's unclear whether or not this BLOCK action registers some sort of negative reputation market against a sender, but I suspect it does.