Microsoft Changes List-Unsubscribe Requirements

Microsoft recently confirmed changes to how they support the list-unsubscribe header in their webmail interfaces, and now only accept the mailto URI. The updated postmaster page states:

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. 

List-unsubscribe, an x-header defined in RFC 2369, is text located within a header that email senders can include to provide a safe way for end users to safely unsubscribe. The list-unsubscribe has been a very valuable tool for the email ecosystem, from consumers to businesses to mailbox providers. Over the past 20 years, consumers have slowly been trained to mistrust unsubscribe links located in the footers of email and spam, as some spammers would use the unsubscribe link to verify that the email address was a valid, active user. Once the spammers knew that, they would send them even more email rather than opting them out. In some cases, spammers would use the link as a way to install malware on an unsuspecting users’ machine.

That still left a need for users to unsubscribe from messages. Past surveys have indicated that over 1 in 5 people use the “report spam” button to unsubscribe from a newsletter, rather than actually clicking on the unsubscribe link in the email. They don’t do this to annoy businesses or because they really think it’s spam, but rather of years of abuse and mistrust from spammers using the unsubscribe link in an email to verify its validity.

So how do the Mailbox providers benefit from the use of list-unsubscribe? Since their users were using the “report spam” button in a way that wasn’t originally intended, businesses would often see inflated complaint rates. This in turn caused false positives with mailbox providers’ spam filters, and flagged opt-in permission-based email as spam. By creating a trusted way for people to unsubscribe, spam complaint rates have been more accurate, and mailbox providers have gotten better at separating spam from graymail. This also explains why both and Gmail use the list-unsubscribe functionality leverage the list-unsubscribe option when for senders with good sending reputations. Neither Google or Microsoft want the list-unsubscribe to be abused by spammers, too.

According to Gmail’s postmaster page, they will honor either the http or the mailto URI:

It’s possible that your users forward mail from other accounts, so we recommend that you:

  • Explicitly indicate the email address subscribed to your list.
  • Support a URL method of unsubscribing from your mailing list (this is beneficial if your mailing list manager can’t tell who is unsubscribing based on the ‘Reply-to:’ address).

While Gmail states that they will honor both, they have stated in the past that they would prefer senders to use a mailto URI, so it’s likely we may see Gmail making a similar move. It’s recommended that senders now focus on only the mailto URI.

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