- 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.
I've seen some folks complain that the list unsubscribe header is bad and that it should be removed, because it makes it too easy for recipients to unsubscribe from a company's marketing email messages. Well, here's a very significant downside that can apply to you if you don't have (or if you remove) the list-unsubscribe header from your email messages.