Jessica, Technically the List-Unsubscribe header should be added to all standard emails. There is a chance @hotmail.com addresses may not see the header added, when we first implemented the header there was a bug in Hotmail/Outlook.com's use of the header so we excluded them originally. I believe that exclusion was rolled back when the bug was fixed. We do not typically add the header to test/sample messages however there is a bug where we see the header added to most test messages. We are not looking to fix this bug because it can be useful sometimes when testing email content. If the header is missing from an email that was sent as a standard part of the email campaign it is a bug. If you wanted to report it through Marketo's Support channel I would recommend grabbing the email headers from those samples you show for deeper review. Here's a Google site on how to find email headers in different providers: Message headers - Gmail Help Thank you! Kiersti
... View more
.png)