Recently, Microsoft has changed its list unsubscribe requirements for Outlook.com. This is noted on the Return Path blog by Melinda Plemel.
The changes with regards to the list unsubscribe are detailed on the updated Outlook.com Postmaster page:
In order to receive unsubscribe feedback, senders must include an RFC2369-compliant List-Unsubscribe header containing a mailto: address. Please note that we only enable this feedback via email, so URIs for other protocols such as http will be ignored. The sender must also have a good reputation, and must act promptly in removing users from their lists. We do not provide unsubscribe feedback to senders when a user unsubscribes from an untrusted message.
A mailto:address is required to receive unsubscribe feedback: URIs from http protocols will be ignored.
How does list unsubscribe work for the end user?
List unsubscribe is used in the email industry to have consumers use a trustworthy way of unsubscribing. Rather than reporting email as spam when people just want to unsubscribe, list unsubscribe allows email users to unsubscribe in a normal fashion from email newsletters. List-unsubscribe is featured in (for example) Outlook.com’s webmail interface. See an example below how it works:
And here’s what the list unsubscribe feature looks like in the Gmail webmail interface:
Gmail still allows both types of URIs to be used (http and mailto), but noted in the past that mailto is preferred.
Read more about the changes to the list unsubscribe requirements by Microsoft on the Return Path website.